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/survey.cpp

@@ -258,7 +258,7 @@ void SurveyConfiguration(nlohmann::json &survey)
 {
 	survey["network"] = _networking ? (_network_server ? "server" : "client") : "no";
 	if (_current_language != nullptr) {
-		survey["language"]["filename"] = _current_language->file.filename().string();
+		survey["language"]["filename"] = FS2OTTD(_current_language->file.filename());
 		survey["language"]["name"] = _current_language->name;
 		survey["language"]["isocode"] = _current_language->isocode;
 	}