Fix default language

BUG: 398245

Following scenario:

I have the de_EN format specified, but no default language configured, so falls back to en_US. But the default function returns the format, even if it does not exist:

sonnet.core: No language dictionaries for the language: "en_DE"

This is also confusing for the users, because en_US is correctly selected in the spellcheck and language dialogs.

The KCM where you configure the languages has a similar logic for the fallback:

      QString lang = formatsConfig.readEntry("LANG", QString());

      if (lang.isEmpty()
          || !KLocalizedString::availableDomainTranslations("plasmashell").contains(lang)) {
          lang = QLocale::system().name();
      }

      if (!KLocalizedString::availableDomainTranslations("plasmashell").contains(lang)) {
          lang = QStringLiteral("en_US");
      }

@ngraham @dfaure @aacid