### Question:
LanguageCache { @Nonnull @VisibleForTesting EChange addLanguage (@Nonnull final String sLanguage) { ValueEnforcer.notNull (sLanguage, "Language"); final String sValidLanguage = LocaleHelper.getValidLanguageCode (sLanguage); ValueEnforcer.isTrue (sValidLanguage != null, () -> "illegal language code '" + sLanguage + "'"); ValueEnforcer.isEqual (sLanguage, sValidLanguage, () -> "invalid casing of '" + sLanguage + "'"); return m_aRWLock.writeLockedGet ( () -> m_aLanguages.addObject (sValidLanguage)); } private  LanguageCache(); static boolean isSilentMode(); static boolean setSilentMode(final boolean bSilentMode); static boolean isInstantiated(); @Nonnull static LanguageCache getInstance(); @Nullable Locale getLanguage(@Nullable final Locale aLanguage); @Nullable Locale getLanguageExt(@Nullable final Locale aLanguage, @Nullable final IMissingLocaleHandler aMissingHandler); @Nullable Locale getLanguage(@Nullable final String sLanguage); @Nullable Locale getLanguageExt(@Nullable final String sLanguage, @Nullable final IMissingLocaleHandler aMissingHandler); @Nonnull @ReturnsMutableCopy ICommonsSet <String> getAllLanguages(); @Nonnull @ReturnsMutableCopy ICommonsSet <Locale> getAllLanguageLocales(); boolean containsLanguage(@Nullable final Locale aLanguage); boolean containsLanguage(@Nullable final String sLanguage); void reinitialize();  }### Answer:
@Test public void testAddLanguage () { assertTrue (LanguageCache.getInstance ().addLanguage ("yx").isChanged ()); }