I've installed Flatpak LO version and there is no way how to get another spell check language eg. Czech.
BTW - I can switch to Czech GUI but not spell checker. I suppose language dictionaries are not bundled in Flatpak repository.
Stephan: what's the plan for non-English spellcheckers?
The plan is that those get bundled with the underlying GNOME runtime. Somewhat inconsistently, org.gnome.Platform//3.20 includes the English hunspell dictionaries, but none for other locales. Probably all of them should be available in org.gnome.Platform.Locale, see upstream issue <https://github.com/flatpak/flatpak/issues/702> "Include hunspell dictionaries in org.gnome.Platform.Locale".