[Regression]: Locale silently ignored on Android — toString() emits "zh_TW" but native forLanguageTag() accepts BCP-47 only

Start in geocoding_android-5.1.0/lib/src/geocoding_android.dart:164 and compare the locale identifier with android/src/main/kotlin/com/baseflow/geocoding/proxies/LocaleProxyApi.kt:17. Check the existing package tests, then verify that country and script locales reach the native API as valid BCP-47 tags and that Android localization is preserved without affecting iOS.

Is there an existing issue for this?

• I have searched the existing issues — found none covering this.

Affected platform(s)

• Android
• iOS

Old behavior

On geocoding_android 4.x, passing a Locale with a country subtag worked: the returned placemarks were localized to that locale. The native side converted the identifier manually (LocaleConverter.java, splitting the string on _), which accepted the underscore form that the Dart side produces.

Current behavior

On geocoding_android 5.x the locale is silently ignored — results come back in the device's default language. The call succeeds, no exception, nothing observable at the call site.

Root cause is a mismatch between the producer and the consumer of the identifier string.

Dart side serializes with toString():

// geocoding_android-5.1.0/lib/src/geocoding_android.dart:164
final Locale? nativeLocale =
    locale != null ? Locale(identifier: locale.toString()) : null;

Flutter's Locale.toString() joins subtags with an underscore — Locale('zh','TW').toString() == "zh_TW".

Native side parses with forLanguageTag():

// geocoding_android/android/src/main/kotlin/com/baseflow/geocoding/proxies/LocaleProxyApi.kt:17
return Locale.forLanguageTag(identifier);

Locale.forLanguageTag accepts BCP-47 only. Per the Javadoc: "If the specified language tag contains any ill-formed subtags, the first such subtag and all following subtags are ignored." zh_TW is a single ill-formed subtag, so the entire tag is dropped and an empty (und) Locale is returned.

Verified on JDK 21:

forLanguageTag("zh_TW")      -> language=""   country=""   toLanguageTag="und"
forLanguageTag("zh-TW")      -> language="zh" country="TW" toLanguageTag="zh-TW"
forLanguageTag("zh_Hant_TW") -> language=""   country=""   toLanguageTag="und"
forLanguageTag("en_US")      -> language=""   country=""   toLanguageTag="und"

Note that GeocoderProxyApi passes this non-null but empty Locale straight to Geocoder(context, locale), so it does not fall back to the null-locale path — the request goes out with an undetermined locale.

This affects every locale carrying a country or script subtag (en_US, pt_BR, zh_Hant_TW, …). A language-only locale such as Locale('ja') happens to still work, since "ja" is already a well-formed tag.

Side note: the README still documents the identifier format as [languageCode]_[countryCode] (underscore) — which is now precisely the form that fails.

Steps to reproduce

1. On Android, set the device language to something other than the locale you are about to request (e.g. device in English).
2. Call placemarkFromCoordinates(lat, lng, locale: const Locale('zh', 'TW')) — any locale with a country subtag reproduces it.
3. Observe the returned Placemark fields: they are in the device language, not the requested one. No error is raised.
4. Repeat with const Locale('zh-TW') (single subtag, so toString() already yields a hyphen) — the result is correctly localized.

Code sample

// Ignored on Android (serializes to "zh_TW" -> forLanguageTag -> und):
final ignored = await Geocoding().placemarkFromCoordinates(
  25.0330, 121.5654,
  locale: const Locale('zh', 'TW'),
);

// Honored (serializes to "zh-TW"):
final honored = await Geocoding().placemarkFromCoordinates(
  25.0330, 121.5654,
  locale: const Locale('zh-TW'),
);

Suggested fix

Serialize with toLanguageTag() instead of toString() on the Dart side:

Locale(identifier: locale.toLanguageTag())   // "zh-TW"

toLanguageTag() emits exactly BCP-47, which is what forLanguageTag expects. One line, no native change required.

iOS is unaffected either way — geocoding_darwin uses Locale(identifier:), and Foundation parses "zh_TW" and "zh-TW" identically (verified on macOS: both yield language=zh region=TW). So switching to toLanguageTag() is safe for both platforms.

Workaround for users

Pass the tag as a single-subtag Dart Locale, so that toString() already produces the hyphenated form:

const Locale('zh-TW')    // toString() == "zh-TW"  -> honored
const Locale('zh', 'TW') // toString() == "zh_TW"  -> silently ignored

Current version

geocoding: 5.0.0 / geocoding_android: 5.1.0 (latest at time of writing) / geocoding_platform_interface: 5.0.0. Still present on main.

Last version without regression

geocoding_android: 4.0.1