### Question:
LocaleHelper { @Nullable public static String getValidLanguageCode (@Nullable final String sCode) { if (StringHelper.hasText (sCode) && (RegExHelper.stringMatchesPattern ("[a-zA-Z]{2,8}", sCode) || isSpecialLocaleCode (sCode))) { return sCode.toLowerCase (CGlobal.LOCALE_FIXED_NUMBER_FORMAT); } return null; } private  LocaleHelper(); @Nonnull static String getLocaleDisplayName(@Nullable final Locale aLocale, @Nonnull final Locale aContentLocale); @Nonnull static String getLocaleNativeDisplayName(@Nonnull final Locale aLocale); @Nonnull @ReturnsMutableCopy static ICommonsMap <Locale, String> getAllLocaleDisplayNames(@Nonnull final Locale aContentLocale); @Nonnull @ReturnsImmutableObject @CodingStyleguideUnaware static List <Locale> getCalculatedLocaleListForResolving(@Nonnull final Locale aLocale); @Nonnull static Locale getLocaleFromString(@Nullable final String sLocaleAsString); @Nullable static Locale getLocaleToUseOrNull(@Nonnull final Locale aRequestLocale, @Nonnull final Collection <Locale> aAvailableLocales); @Nullable static Locale getLocaleToUseOrFallback(@Nonnull final Locale aRequestLocale,
                                                 @Nonnull final Collection <Locale> aAvailableLocales,
                                                 @Nullable final Locale aFallback); static boolean isSpecialLocale(@Nullable final Locale aLocale); static boolean isSpecialLocaleCode(@Nullable final String sLocale); @Nullable static String getValidLanguageCode(@Nullable final String sCode); @Nullable static String getValidCountryCode(@Nullable final String sCode); @Nonnull static EChange clearCache(); static final char LOCALE_SEPARATOR; static final String STR_ALL; static final Locale LOCALE_ALL; static final String STR_INDEPENDENT; static final Locale LOCALE_INDEPENDENT; }### Answer:
@Test public void testGetValidLanguageCode () { assertNull (LocaleHelper.getValidLanguageCode (null)); assertNull (LocaleHelper.getValidLanguageCode ("")); assertNull (LocaleHelper.getValidLanguageCode ("1")); assertNull (LocaleHelper.getValidLanguageCode ("a")); assertNull (LocaleHelper.getValidLanguageCode (" ")); assertNull (LocaleHelper.getValidLanguageCode ("_")); assertNull (LocaleHelper.getValidLanguageCode ("_de")); assertNull (LocaleHelper.getValidLanguageCode ("d.e")); assertNull (LocaleHelper.getValidLanguageCode ("d e")); assertNull (LocaleHelper.getValidLanguageCode ("abcdefghi")); assertEquals ("de", LocaleHelper.getValidLanguageCode ("De")); assertEquals ("abc", LocaleHelper.getValidLanguageCode ("ABC")); assertEquals ("en", LocaleHelper.getValidLanguageCode ("en")); assertEquals ("abcdefgh", LocaleHelper.getValidLanguageCode ("abcDefgh")); }