diff --git a/pkg/i18n/checks_test.go b/pkg/i18n/checks_test.go
new file mode 100644
index 0000000..19cdbc4
--- /dev/null
+++ b/pkg/i18n/checks_test.go
@@ -0,0 +1,257 @@
+package i18n
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsVerbFormObject(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    map[string]any
+		expected bool
+	}{
+		{
+			name:     "has base only",
+			input:    map[string]any{"base": "run"},
+			expected: true,
+		},
+		{
+			name:     "has past only",
+			input:    map[string]any{"past": "ran"},
+			expected: true,
+		},
+		{
+			name:     "has gerund only",
+			input:    map[string]any{"gerund": "running"},
+			expected: true,
+		},
+		{
+			name:     "has all verb forms",
+			input:    map[string]any{"base": "run", "past": "ran", "gerund": "running"},
+			expected: true,
+		},
+		{
+			name:     "empty map",
+			input:    map[string]any{},
+			expected: false,
+		},
+		{
+			name:     "plural object not verb",
+			input:    map[string]any{"one": "item", "other": "items"},
+			expected: false,
+		},
+		{
+			name:     "unrelated keys",
+			input:    map[string]any{"foo": "bar", "baz": "qux"},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isVerbFormObject(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestIsNounFormObject(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    map[string]any
+		expected bool
+	}{
+		{
+			name:     "has gender",
+			input:    map[string]any{"gender": "masculine", "one": "file", "other": "files"},
+			expected: true,
+		},
+		{
+			name:     "gender only",
+			input:    map[string]any{"gender": "feminine"},
+			expected: true,
+		},
+		{
+			name:     "no gender",
+			input:    map[string]any{"one": "item", "other": "items"},
+			expected: false,
+		},
+		{
+			name:     "empty map",
+			input:    map[string]any{},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isNounFormObject(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestHasPluralCategories(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    map[string]any
+		expected bool
+	}{
+		{
+			name:     "has zero",
+			input:    map[string]any{"zero": "none", "one": "one", "other": "many"},
+			expected: true,
+		},
+		{
+			name:     "has two",
+			input:    map[string]any{"one": "one", "two": "two", "other": "many"},
+			expected: true,
+		},
+		{
+			name:     "has few",
+			input:    map[string]any{"one": "one", "few": "few", "other": "many"},
+			expected: true,
+		},
+		{
+			name:     "has many",
+			input:    map[string]any{"one": "one", "many": "many", "other": "other"},
+			expected: true,
+		},
+		{
+			name:     "has all categories",
+			input:    map[string]any{"zero": "0", "one": "1", "two": "2", "few": "few", "many": "many", "other": "other"},
+			expected: true,
+		},
+		{
+			name:     "only one and other",
+			input:    map[string]any{"one": "item", "other": "items"},
+			expected: false,
+		},
+		{
+			name:     "empty map",
+			input:    map[string]any{},
+			expected: false,
+		},
+		{
+			name:     "unrelated keys",
+			input:    map[string]any{"foo": "bar"},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := hasPluralCategories(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestIsPluralObject(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    map[string]any
+		expected bool
+	}{
+		{
+			name:     "one and other",
+			input:    map[string]any{"one": "item", "other": "items"},
+			expected: true,
+		},
+		{
+			name:     "all CLDR categories",
+			input:    map[string]any{"zero": "0", "one": "1", "two": "2", "few": "few", "many": "many", "other": "other"},
+			expected: true,
+		},
+		{
+			name:     "only other",
+			input:    map[string]any{"other": "items"},
+			expected: true,
+		},
+		{
+			name:     "empty map",
+			input:    map[string]any{},
+			expected: false,
+		},
+		{
+			name:     "nested map is not plural",
+			input:    map[string]any{"one": "item", "other": map[string]any{"nested": "value"}},
+			expected: false,
+		},
+		{
+			name:     "unrelated keys",
+			input:    map[string]any{"foo": "bar", "baz": "qux"},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isPluralObject(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestMessageIsPlural(t *testing.T) {
+	tests := []struct {
+		name     string
+		msg      Message
+		expected bool
+	}{
+		{
+			name:     "has zero",
+			msg:      Message{Zero: "none"},
+			expected: true,
+		},
+		{
+			name:     "has one",
+			msg:      Message{One: "item"},
+			expected: true,
+		},
+		{
+			name:     "has two",
+			msg:      Message{Two: "items"},
+			expected: true,
+		},
+		{
+			name:     "has few",
+			msg:      Message{Few: "a few"},
+			expected: true,
+		},
+		{
+			name:     "has many",
+			msg:      Message{Many: "lots"},
+			expected: true,
+		},
+		{
+			name:     "has other",
+			msg:      Message{Other: "items"},
+			expected: true,
+		},
+		{
+			name:     "has all",
+			msg:      Message{Zero: "0", One: "1", Two: "2", Few: "few", Many: "many", Other: "other"},
+			expected: true,
+		},
+		{
+			name:     "text only",
+			msg:      Message{Text: "hello"},
+			expected: false,
+		},
+		{
+			name:     "empty message",
+			msg:      Message{},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := tt.msg.IsPlural()
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
diff --git a/pkg/i18n/compose.go b/pkg/i18n/compose.go
index dfabbe0..1153439 100644
--- a/pkg/i18n/compose.go
+++ b/pkg/i18n/compose.go
@@ -94,41 +94,41 @@ func (s *Subject) IsPlural() bool {
 	return s != nil && s.count != 1
 }
 
-// GetCount returns the count value.
-func (s *Subject) GetCount() int {
+// CountValue returns the count value.
+func (s *Subject) CountValue() int {
 	if s == nil {
 		return 1
 	}
 	return s.count
 }
 
-// GetGender returns the grammatical gender.
-func (s *Subject) GetGender() string {
+// GenderValue returns the grammatical gender.
+func (s *Subject) GenderValue() string {
 	if s == nil {
 		return ""
 	}
 	return s.gender
 }
 
-// GetLocation returns the location context.
-func (s *Subject) GetLocation() string {
+// Location returns the location context.
+func (s *Subject) Location() string {
 	if s == nil {
 		return ""
 	}
 	return s.location
 }
 
-// GetNoun returns the noun type.
-func (s *Subject) GetNoun() string {
+// NounValue returns the noun type.
+func (s *Subject) NounValue() string {
 	if s == nil {
 		return ""
 	}
 	return s.Noun
 }
 
-// GetFormality returns the formality level.
+// FormalityValue returns the formality level.
 // Returns FormalityNeutral if not explicitly set.
-func (s *Subject) GetFormality() Formality {
+func (s *Subject) FormalityValue() Formality {
 	if s == nil {
 		return FormalityNeutral
 	}
diff --git a/pkg/i18n/compose_test.go b/pkg/i18n/compose_test.go
index 3b8276f..c378e8d 100644
--- a/pkg/i18n/compose_test.go
+++ b/pkg/i18n/compose_test.go
@@ -33,26 +33,26 @@ func TestSubject_Good(t *testing.T) {
 
 	t.Run("with count", func(t *testing.T) {
 		s := S("file", "*.go").Count(5)
-		assert.Equal(t, 5, s.GetCount())
+		assert.Equal(t, 5, s.CountValue())
 		assert.True(t, s.IsPlural())
 	})
 
 	t.Run("with gender", func(t *testing.T) {
 		s := S("user", "alice").Gender("female")
-		assert.Equal(t, "female", s.GetGender())
+		assert.Equal(t, "female", s.GenderValue())
 	})
 
 	t.Run("with location", func(t *testing.T) {
 		s := S("file", "config.yaml").In("workspace")
-		assert.Equal(t, "workspace", s.GetLocation())
+		assert.Equal(t, "workspace", s.Location())
 	})
 
 	t.Run("chained methods", func(t *testing.T) {
 		s := S("repo", "core-php").Count(3).Gender("neuter").In("organisation")
-		assert.Equal(t, "repo", s.GetNoun())
-		assert.Equal(t, 3, s.GetCount())
-		assert.Equal(t, "neuter", s.GetGender())
-		assert.Equal(t, "organisation", s.GetLocation())
+		assert.Equal(t, "repo", s.NounValue())
+		assert.Equal(t, 3, s.CountValue())
+		assert.Equal(t, "neuter", s.GenderValue())
+		assert.Equal(t, "organisation", s.Location())
 	})
 }
 
@@ -104,10 +104,10 @@ func TestSubject_IsPlural(t *testing.T) {
 func TestSubject_Getters(t *testing.T) {
 	t.Run("nil safety", func(t *testing.T) {
 		var s *Subject
-		assert.Equal(t, "", s.GetNoun())
-		assert.Equal(t, 1, s.GetCount())
-		assert.Equal(t, "", s.GetGender())
-		assert.Equal(t, "", s.GetLocation())
+		assert.Equal(t, "", s.NounValue())
+		assert.Equal(t, 1, s.CountValue())
+		assert.Equal(t, "", s.GenderValue())
+		assert.Equal(t, "", s.Location())
 	})
 }
 
@@ -193,31 +193,31 @@ func TestNewTemplateData(t *testing.T) {
 func TestSubject_Formality(t *testing.T) {
 	t.Run("default is neutral", func(t *testing.T) {
 		s := S("user", "name")
-		assert.Equal(t, FormalityNeutral, s.GetFormality())
+		assert.Equal(t, FormalityNeutral, s.FormalityValue())
 		assert.False(t, s.IsFormal())
 		assert.False(t, s.IsInformal())
 	})
 
 	t.Run("Formal()", func(t *testing.T) {
 		s := S("user", "name").Formal()
-		assert.Equal(t, FormalityFormal, s.GetFormality())
+		assert.Equal(t, FormalityFormal, s.FormalityValue())
 		assert.True(t, s.IsFormal())
 	})
 
 	t.Run("Informal()", func(t *testing.T) {
 		s := S("user", "name").Informal()
-		assert.Equal(t, FormalityInformal, s.GetFormality())
+		assert.Equal(t, FormalityInformal, s.FormalityValue())
 		assert.True(t, s.IsInformal())
 	})
 
 	t.Run("Formality() explicit", func(t *testing.T) {
 		s := S("user", "name").Formality(FormalityFormal)
-		assert.Equal(t, FormalityFormal, s.GetFormality())
+		assert.Equal(t, FormalityFormal, s.FormalityValue())
 	})
 
 	t.Run("nil safety", func(t *testing.T) {
 		var s *Subject
-		assert.Equal(t, FormalityNeutral, s.GetFormality())
+		assert.Equal(t, FormalityNeutral, s.FormalityValue())
 		assert.False(t, s.IsFormal())
 		assert.False(t, s.IsInformal())
 	})
diff --git a/pkg/i18n/mutate_test.go b/pkg/i18n/mutate_test.go
new file mode 100644
index 0000000..14009f7
--- /dev/null
+++ b/pkg/i18n/mutate_test.go
@@ -0,0 +1,246 @@
+package i18n
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFlatten(t *testing.T) {
+	tests := []struct {
+		name     string
+		prefix   string
+		data     map[string]any
+		expected map[string]Message
+	}{
+		{
+			name:   "simple string",
+			prefix: "",
+			data:   map[string]any{"hello": "world"},
+			expected: map[string]Message{
+				"hello": {Text: "world"},
+			},
+		},
+		{
+			name:   "nested object",
+			prefix: "",
+			data: map[string]any{
+				"cli": map[string]any{
+					"success": "Done",
+					"error":   "Failed",
+				},
+			},
+			expected: map[string]Message{
+				"cli.success": {Text: "Done"},
+				"cli.error":   {Text: "Failed"},
+			},
+		},
+		{
+			name:   "with prefix",
+			prefix: "app",
+			data:   map[string]any{"key": "value"},
+			expected: map[string]Message{
+				"app.key": {Text: "value"},
+			},
+		},
+		{
+			name:   "deeply nested",
+			prefix: "",
+			data: map[string]any{
+				"a": map[string]any{
+					"b": map[string]any{
+						"c": "deep value",
+					},
+				},
+			},
+			expected: map[string]Message{
+				"a.b.c": {Text: "deep value"},
+			},
+		},
+		{
+			name:   "plural object",
+			prefix: "",
+			data: map[string]any{
+				"items": map[string]any{
+					"one":   "{{.Count}} item",
+					"other": "{{.Count}} items",
+				},
+			},
+			expected: map[string]Message{
+				"items": {One: "{{.Count}} item", Other: "{{.Count}} items"},
+			},
+		},
+		{
+			name:   "full CLDR plural",
+			prefix: "",
+			data: map[string]any{
+				"files": map[string]any{
+					"zero":  "no files",
+					"one":   "one file",
+					"two":   "two files",
+					"few":   "a few files",
+					"many":  "many files",
+					"other": "{{.Count}} files",
+				},
+			},
+			expected: map[string]Message{
+				"files": {
+					Zero:  "no files",
+					One:   "one file",
+					Two:   "two files",
+					Few:   "a few files",
+					Many:  "many files",
+					Other: "{{.Count}} files",
+				},
+			},
+		},
+		{
+			name:   "mixed content",
+			prefix: "",
+			data: map[string]any{
+				"simple": "text",
+				"plural": map[string]any{
+					"one":   "singular",
+					"other": "plural",
+				},
+				"nested": map[string]any{
+					"child": "nested value",
+				},
+			},
+			expected: map[string]Message{
+				"simple":       {Text: "text"},
+				"plural":       {One: "singular", Other: "plural"},
+				"nested.child": {Text: "nested value"},
+			},
+		},
+		{
+			name:     "empty data",
+			prefix:   "",
+			data:     map[string]any{},
+			expected: map[string]Message{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			out := make(map[string]Message)
+			flatten(tt.prefix, tt.data, out)
+			assert.Equal(t, tt.expected, out)
+		})
+	}
+}
+
+func TestFlattenWithGrammar(t *testing.T) {
+	t.Run("extracts verb forms", func(t *testing.T) {
+		data := map[string]any{
+			"gram": map[string]any{
+				"verb": map[string]any{
+					"run": map[string]any{
+						"base":   "run",
+						"past":   "ran",
+						"gerund": "running",
+					},
+				},
+			},
+		}
+		out := make(map[string]Message)
+		grammar := &GrammarData{
+			Verbs: make(map[string]VerbForms),
+			Nouns: make(map[string]NounForms),
+		}
+		flattenWithGrammar("", data, out, grammar)
+
+		assert.Contains(t, grammar.Verbs, "run")
+		assert.Equal(t, "ran", grammar.Verbs["run"].Past)
+		assert.Equal(t, "running", grammar.Verbs["run"].Gerund)
+	})
+
+	t.Run("extracts noun forms", func(t *testing.T) {
+		data := map[string]any{
+			"gram": map[string]any{
+				"noun": map[string]any{
+					"file": map[string]any{
+						"one":    "file",
+						"other":  "files",
+						"gender": "neuter",
+					},
+				},
+			},
+		}
+		out := make(map[string]Message)
+		grammar := &GrammarData{
+			Verbs: make(map[string]VerbForms),
+			Nouns: make(map[string]NounForms),
+		}
+		flattenWithGrammar("", data, out, grammar)
+
+		assert.Contains(t, grammar.Nouns, "file")
+		assert.Equal(t, "file", grammar.Nouns["file"].One)
+		assert.Equal(t, "files", grammar.Nouns["file"].Other)
+		assert.Equal(t, "neuter", grammar.Nouns["file"].Gender)
+	})
+
+	t.Run("extracts articles", func(t *testing.T) {
+		data := map[string]any{
+			"gram": map[string]any{
+				"article": map[string]any{
+					"indefinite": map[string]any{
+						"default": "a",
+						"vowel":   "an",
+					},
+					"definite": "the",
+				},
+			},
+		}
+		out := make(map[string]Message)
+		grammar := &GrammarData{
+			Verbs: make(map[string]VerbForms),
+			Nouns: make(map[string]NounForms),
+		}
+		flattenWithGrammar("", data, out, grammar)
+
+		assert.Equal(t, "a", grammar.Articles.IndefiniteDefault)
+		assert.Equal(t, "an", grammar.Articles.IndefiniteVowel)
+		assert.Equal(t, "the", grammar.Articles.Definite)
+	})
+
+	t.Run("extracts punctuation rules", func(t *testing.T) {
+		data := map[string]any{
+			"gram": map[string]any{
+				"punct": map[string]any{
+					"label":    ":",
+					"progress": "...",
+				},
+			},
+		}
+		out := make(map[string]Message)
+		grammar := &GrammarData{
+			Verbs: make(map[string]VerbForms),
+			Nouns: make(map[string]NounForms),
+		}
+		flattenWithGrammar("", data, out, grammar)
+
+		assert.Equal(t, ":", grammar.Punct.LabelSuffix)
+		assert.Equal(t, "...", grammar.Punct.ProgressSuffix)
+	})
+
+	t.Run("nil grammar skips extraction", func(t *testing.T) {
+		data := map[string]any{
+			"gram": map[string]any{
+				"verb": map[string]any{
+					"run": map[string]any{
+						"past":   "ran",
+						"gerund": "running",
+					},
+				},
+			},
+			"simple": "text",
+		}
+		out := make(map[string]Message)
+		flattenWithGrammar("", data, out, nil)
+
+		// Without grammar, verb forms are recursively processed as nested objects
+		assert.Contains(t, out, "simple")
+		assert.Equal(t, "text", out["simple"].Text)
+	})
+}