dickesKind: I found the solution by some more research:
The language Banner Saga starts the game in depends on the location you are at.
If you experience a reset of your chosen language you have to edit the file "tbs_config.ini".
You can find this file in the subfolder "win32".
After opening the file with an editor you have to find the line representing your language and
edit the language after the equals sign.
In my case the language was always set to German as I am from Germany. I edited the file so that the language when starting the game is English.
locale.en=en
locale.zh=zh
locale.cs=cs
locale.ru=ru
locale.ko=ko
locale.de=en <--
locale.es=es
locale.jp=jp
locale.ja=jp
locale.ja-jp=jp
locale.fr=fr
locale.it=it
locale.pl=pl
locale.pt=pt-br
If you start the Game in Russian but want the game to start with German language I would have to edit it like this: "locale.ru=de".
If you are from France and want it Russian you would have to edit it like this "locale.fr=ru".
I hope this helps.
We really appreciate you finding and posting the workaround for this. Thanks so much!