How to indicate obscurity in a translated quotation?

After all, it's not what was actually written but my translation, so I feel like this isn't really an option.

You seem to have unconsciously ruled out what I would consider the best option: leave the phrase in question untranslated with a footnote which gives the literal translation and says that the phrase's meaning is unclear but irrelevant.

In language-related papers, it is common to do all of these options:

• give the untranslated version
• give the literal translation
• give the semantically closest translation in English

Of course it depends how important this particular sentence is to your work. But if you want to mention it, you should at least give the original sentence and a translation in my opinion.

Give the loose translation in brackets. For instance, if the original says "This happens once in a blue moon", then give the translation as "This happens [very infrequently]". If you want to be complete, give a footnote with the original German and literal translation.

So-and-so says "this happens [very infrequently]^[1]".

[1]"alle Jubeljahre", literally "once in a blue moon", an idiom expressing extremely low frequency

(Obviously, this answer is pretending, for the sake of an example, that "alle Jubeljahre" is German idiom that literally translates to "once in a blue moon" but means "infrequently", and is not asserting that this is the case in reality.)