Continuing http://tex.stackexchange.com/questions/325453/flyspell-babel-warning-no-dictionary-installed-for-foreignlanguage and How can I make AUCTeX spell check in the language specified by babel and csquotes macros?, let's take as an example the following file in UTF-8:
Außenwirkung
Nutzungsfälle
\foreignlanguage{USenglish}{behavior}
\foreignlanguage{UKenglish}{behaviour}
%%% Local Variables:
%%% mode: latex
%%% ispell-local-dictionary: "german-new8"
%%% coding: utf-8
%%% End:
It is \input from another main file, which contains \usepackage[USenglish,british,french,norsk,dutch,latin,german,ngerman]{babel}
I typically use emacs 26.1 for editing with aspell 0.60.7-20110707 for spellchecking (but I'm ready to change my habits if there's a better solution for spellchecking). The German words are checked without problems (of course, you might wish to add them to your local dictionary if they are not in the global one). The English words are problematic: each time I run M-x ispell-buffer, they show up as wrong. Is there any way to automatically check also the \foreignlanguage{...}{...} parts using the proper dictionary? As of now, flyspell-babel is unfortunately dead or at least seems dead to me (cf. http://texwelt.de/wissen/fragen/25319/wie-bringe-ich-flyspell-babel-zum-laufen).
Crossposts: http://latex.org/forum/viewtopic.php?f=54&t=32702, http://golatex.de/viewtopic,p,107348.html
hunspell, which I call from R withpythontex. It is not the simplest of solutions, but works (mostly). My spellchecking options were limited because of language I am working with; OS windows and my employer IT rules. For english, there are also other options - through lua or perl; but none of them I have tested. – Tomáš Kruliš Jul 11 '20 at 16:03