My suggestion is to enter the text already encoded as UTF-8 and then to instruct vim
to accept other than, or additional languages to English for the spell check.
With the following example containing two intentional errors:
Kiti didieji miestai: Kaunas, Klaipėda, Šiauliai, Panevėžys.
This is correct, but here is an errror
L'école se situe en face de l'hôtel Bienvennu
the default correction by set spell
is the following

This may be changed, e.g. to check only for French by :set spell spelllang=fr
:

or to check simultaneously for multiple languages. Altogether with Lithuanian, the command here is :set spell spelllang=lt,fr,en
. Note the list is without a blank after the comma:

This is how to add new dictionaries to the checker: As an example, for Spanish, call set spell spelllang=es
. This opens a dialogue to fetch the corresponding .spl
with wget
to be stored in the path of .vim/spell
. There are languages with multiple definitions, e.g. British English / US English.
Jarník
(i.e., using utf-8 encoding in your files), it works for me out-of-the box withvimtex
(and I suppose anyvim
plugin).\v
) from the keyboard is much more convoluted. I guess I should have mentioned it in the question, but that is why I formulated it in terms of LaTeX-style diacritics, so to speak.