When i install a GTK program its in default English, But my whole system is in Polish. And i do know for a fact that the things i want to use on GTK have a Polish translation.
$ locale
LANG=pl_PL.UTF-8
LANGUAGE=pl
LC_CTYPE="pl_PL.UTF-8"
LC_NUMERIC="pl_PL.UTF-8"
LC_TIME="pl_PL.UTF-8"
LC_COLLATE="pl_PL.UTF-8"
LC_MONETARY="pl_PL.UTF-8"
LC_MESSAGES="pl_PL.UTF-8"
LC_PAPER="pl_PL.UTF-8"
LC_NAME="pl_PL.UTF-8"
LC_ADDRESS="pl_PL.UTF-8"
LC_TELEPHONE="pl_PL.UTF-8"
LC_MEASUREMENT="pl_PL.UTF-8"
LC_IDENTIFICATION="pl_PL.UTF-8"
LC_ALL=
I am on Kubuntu 19.10