Sorry that you didn't understand my idea.
I hope that you have installed Gk Contact plugin, so you should find those files
administrator\language\en-GB\en-GB.plg_system_gkcontact.ini
administrator\language\en-GB\en-GB.plg_system_gkcontact.sys.ini
copy them to your computer from server
rename filename changing file prefix, for example, if you're French
FROM
en-GB.plg_system_gkcontact.ini
TO
fr-FR.plg_system_gkcontact.ini
Now you can translate it /use UTF-8 if your language have special chars/
After translation process copy to
administrator\language\fr-FR\
That's all, no magic.