add ToProviderFormat for language code conversion

internal/language/language.go

@@ -118,3 +118,49 @@ func IsValidCode(code string) bool {
 	_, ok := codeIndex[code]
 	return ok
 }
+
+// ToProviderFormat converts a canonical language code to the format expected by a specific provider.
+// Each provider may have different expectations:
+//   - whisper-cpp: uses standard codes like 'en', 'auto' for auto-detect
+//   - openai: uses standard codes like 'en', empty string for auto-detect
+//   - groq: same as openai (OpenAI-compatible)
+//   - mistral: same as openai (OpenAI-compatible)
+//   - deepgram: uses locale codes like 'en-US', 'es' for Spanish
+//   - elevenlabs: uses standard codes or full names depending on API version
+func ToProviderFormat(code string, providerName string) string {
+	// handle auto-detect (empty code)
+	if code == "" {
+		switch providerName {
+		case "whisper-cpp":
+			return "auto"
+		default:
+			// most providers use empty string or omit the parameter
+			return ""
+		}
+	}
+
+	switch providerName {
+	case "deepgram":
+		// deepgram prefers locale codes for some languages
+		return toDeepgramFormat(code)
+	default:
+		// whisper-cpp, openai, groq, mistral, elevenlabs use standard codes
+		return code
+	}
+}
+
+// toDeepgramFormat maps standard codes to Deepgram's preferred format
+func toDeepgramFormat(code string) string {
+	// deepgram uses locale codes for English variants, standard for most others
+	deepgramMappings := map[string]string{
+		"en": "en-US",
+		"es": "es",    // Spanish uses base code
+		"pt": "pt-BR", // Portuguese defaults to Brazilian
+		"zh": "zh-CN", // Chinese defaults to Simplified
+	}
+
+	if mapped, ok := deepgramMappings[code]; ok {
+		return mapped
+	}
+	return code
+}

internal/language/language_test.go

@@ -117,3 +117,49 @@ func TestAuto(t *testing.T) {
 		t.Errorf("Auto.Name = %q, want 'Auto-detect'", Auto.Name)
 	}
 }
+
+func TestToProviderFormat(t *testing.T) {
+	tests := []struct {
+		code     string
+		provider string
+		want     string
+	}{
+		// whisper-cpp
+		{"en", "whisper-cpp", "en"},
+		{"es", "whisper-cpp", "es"},
+		{"", "whisper-cpp", "auto"},
+
+		// openai
+		{"en", "openai", "en"},
+		{"", "openai", ""},
+
+		// groq (openai-compatible)
+		{"en", "groq", "en"},
+		{"", "groq", ""},
+
+		// mistral (openai-compatible)
+		{"en", "mistral", "en"},
+		{"", "mistral", ""},
+
+		// deepgram (uses locale codes)
+		{"en", "deepgram", "en-US"},
+		{"es", "deepgram", "es"},
+		{"pt", "deepgram", "pt-BR"},
+		{"zh", "deepgram", "zh-CN"},
+		{"fr", "deepgram", "fr"}, // no special mapping, passthrough
+		{"", "deepgram", ""},
+
+		// elevenlabs
+		{"en", "elevenlabs", "en"},
+		{"", "elevenlabs", ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.code+"_"+tt.provider, func(t *testing.T) {
+			got := ToProviderFormat(tt.code, tt.provider)
+			if got != tt.want {
+				t.Errorf("ToProviderFormat(%q, %q) = %q, want %q", tt.code, tt.provider, got, tt.want)
+			}
+		})
+	}
+}