Multilanguage: Add utility to get language names consistently

There are multiple places that language codes are converted
to language names each doing it a different way. To make
this consistent, we need to have a single implementation
for it.

Change-Id: I2aeaf5b7c9203e8a41a61420f4c98e1fce491db5
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/utils/stringutils.h

@@ -121,4 +121,7 @@ QTCREATOR_UTILS_EXPORT QString formatElapsedTime(qint64 elapsed);
  * into account and handling them to disallow matching a wildcard characters.
  */
 QTCREATOR_UTILS_EXPORT QString wildcardToRegularExpression(const QString &original);
+
+QTCREATOR_UTILS_EXPORT QString languageNameFromLanguageCode(const QString &languageCode);
+
 } // namespace Utils