TIL: The #Signal desktop app on #Linux supports #spellchecking in multiple languages, but there is no selection UI. You can, however, list multiple locales in the LANGUAGE environment variable, delimited by colons. The first one selects the interface language. For example:
LANGUAGE=en_US:de_DE
will give you a US English interface and spell checking in US English and German (Germany).
No need to install anything, Signal will download dictionaries on first use.