First
Please do all the translations from "BO / tools / translations"
in your database there are many tables like "something_lang"
yes you can make the translations from there but its dangerous
Second
If you want to make changes to the header you must find the header.tpl
(when you make changes, first of all make a copy of your original .tpl file to a folder)
Third
if a theme works, then everything is ok
in Summary
Must try, must spend a lot of time and always strive to become better