Limited demo of a paid app. The list of languages on the site is limited to "English, German, Spanish, French, Portuguese, and Dutch", which may make it appear that spell checking is not available in other languages. You cannot choose "EN-GB" or "EN-US", there is only some kind of "English" with the British flag. So the American language is not supported? Uses a third party sync, not a Mozilla account.
You can find the list of supported languages at https://dev.languagetool.org/languages. The languages you listed are the languages with extra rules for premium users. EN-US and EN-GB are both supported, using the American or British flag as their icons.
