Can you point out a specific issue in any of the screenshots you have posted? Looks like the language suffix is correctly added for every single item in the
New Names list via one of the code paths in your custom format.

All non-subtitles file do not have
(and should not have) subtitle language suffixes.

All subtitle language suffixes seem to be correct, where
.eng is detected from the original file via
{subt} and
.ger or
.ger.forced is added generically for files via your custom code in cases where
{subt} is undefined.
{subt} won't work for image-based subtitles
(i.e. idx / sub pairs) because FileBot does not do OCR. The subtitle file must already have a language suffix so FileBot can detect the language based on that, or be text-based so that FileBot can look at the text and detect the language based on that. If
{subt} doesn't work, then your format will just add
.ger or
.ger.forced - depending on the file at hand - for any and all subtitle files.

I notice strange things in the dark mode screenshot you first posted, but I'm fairly sure that you're just using a different format which forces
.ger for all subtitle files there. You can inspect different aspects of your format code in
Format Editor like so. See the value of
{fn}, see what
{subt} returns, see what your custom detection code does, etc.
EDIT:

Are you perhaps having trouble specifically with the
Edit Name feature?
(independent and unrelated to custom formatting)

If you want German movie names names, then manually typing them is not the way to go. You'll want to set
Preferences ➔ Language ➔ German if you want German movie names.
EDIT 2:
I see. If you first match files up, everything works. But if you then manually force a different file name for an individual item, FileBot will apply the updated name to all subsequent items that have the same movie match. I can see how that's not the behaviour we want in this particular corner case. I'll look into it.
There might not be a good solution. You definitely want the current behaviour
(i.e. Edit Name on the *.mkv changes the file name on all the other items) because you don't wanna enter the file name for every single item. If you use
Edit Name then you just need to do it top-down and edit things as needed
(sometimes too much, but you can edit that too when you get to that row). Notably, once you click on
Edit Name then your format becomes irrelevant.
That said, you should never need
Edit Name in the first place, and always preemptively generate the correct file paths you want via your custom format, not by manually entering things.