While trying to run RStudio installer on user
Jakub Poznański, an NSIS error was occurring all the time (error launching installer). Other programs have been launching rather properly.
None of the relevant positions in the FAQ about NSIS helped, not even /NCRC in command prompt.
On the other hand, while launching from user
admin, the installer worked just fine.
It does seem that NSIS supports at least some diacritics, so it appears that the problem is with RStudio itself.
Windows 10 Pro 20H2 (English)
Running RStudio 1.4.1106 (free version)
Thanks for your help!