Thank you. Strangely, when I experimented with FRED, my text saved in the mission.
Not strange at all.
The way translation works is as follows:
The mission file contains a "default language" version of the mission (English) and every bit of text in that mission file has an ID number.
When you play the game in a different language, the game reads that ID number and looks up the appropriate translation using chosen language and this ID from tstrings.tbl.
If it finds one, it shows the translation, and if it doesn't, it shows the "default language" string instead.
So the reason karajorma said that you shouldn't be editing the mission file is because that would actually delete the English version!
In order to
add a new language you have to edit strings.tbl and tstrings.tbl instead.
That said, you're absolutely right that the first step is getting a suitable font.