@Test
@Config(sdk = Config.ALL_SDKS)
public void normalizeLanguageCode_keepsUndefinedTagsUnchanged() {

  assertThat(Util.normalizeLanguageCode(null)).isNull();
  assertThat(Util.normalizeLanguageCode("")).isEmpty();
  assertThat(Util.normalizeLanguageCode("und")).isEqualTo("und");
  assertThat(Util.normalizeLanguageCode("DoesNotExist")).isEqualTo("doesnotexist");
}