Fix #12648: Ensure all uses of std::filesystem::path use native encoding. (#12650)

std::filesystem::path does not expect UTF-8 strings, so they must be converted to native format first (and back to utf-8 after.)

src/game/game_text.cpp

@@ -386,7 +386,7 @@ void ReconsiderGameScriptLanguage()
 {
 	if (_current_data == nullptr) return;
 
-	std::string language = _current_language->file.stem().string();
+	std::string language = FS2OTTD(_current_language->file.stem());
 	for (auto &p : _current_data->compiled_strings) {
 		if (p.language == language) {
 			_current_data->cur_language = &p;