How do you mark a translator's note?

A Google Books search seems to confirm that the usual abbreviation for “translator's note” is indeed TN. The first time you use it, you may do so in full, so that the abbreviation is clear later.

In your particular context, however, it might simply be possible to state the translation inside the parentheses. Using the German Länder as an example, it might give:

Many Länder (“provinces”) have different regulations on the issue

which I think is quite clear. You may even omit the quotes, I think.

The word woord ("word") ...

The word Wort (German for "word") ...

The word mot (Eng. "word") ...

All three choices would be fine in most texts. It may depend on your audience how much explanation the translation requires, whether the language of origin should be mentioned, etc. If your audience is fairly educated, you might be better off without translations of common foreign words, as your readers will not doubt know mot and Wort and might feel patronised—though few would probably know Dutch woord, unless it is obvious from context.

I've seen many people mark a translator's note with "t/n" in almost all the comics I've read. You could try that.

I agree that using first time 'translator's note' in full will clear later its abbreviation as t/n.