rafalcodesprinters

@thomasplevy great you have found the root cause of the problem.

Unfortunately, it does not seem to help when I use “safe” location, already tried that before.

Just see example showing one of my modified translation (hint in account registration form). I have set it in *.po/*.mo file in “safe” location like this:
$ pwd
/var/www/el/wp-content/languages/lifterlms
$ ls -la lifterlms*
-rw-rw-r– 1 www-data www-data 164755 Jul 14 14:31 lifterlms-pl_PL.mo
-rw-rw-r– 1 www-data www-data 365968 Jul 14 14:31 lifterlms-pl_PL.po
$ sed -n -e 842,844p lifterlms-pl_PL.po
#: includes/class.llms.person.handler.php:158
msgid “Apartment, suite, or unit”
msgstr “Numer budynku / mieszkanie”

But when I open the registration form, I still see the translation from the following files:
$ pwd
/var/www/el/wp-content/languages/plugins
$ ls -la lifterlms*
-rw-r–r– 1 www-data www-data 183095 Jul 12 11:40 lifterlms-pl_PL.mo
-rw-r–r– 1 www-data www-data 361784 Jul 12 11:40 lifterlms-pl_PL.po
$ sed -n -e 5350,5352p lifterlms-pl_PL.po
#: includes/class.llms.person.handler.php:158
msgid “Apartment, suite, or unit”
msgstr “Mieszkanie, apartament, lub jednostka”

Apparently, “safe” location is either ignored or translations from there gets overwritten anyway.