I have this little thing with some of my L2 languages that I know that I can understand them in their own language, but not being able to "translate" it over to my L1 and understand it there. Is there a term for being able to understand an L2 and comprehend it as a L2 without being able to translate it back to L1?
I tend to say "doesn't translate well", however a term that fits the bill is Realia:
In translation, Realia (plural noun) are words and expressions for culture-specific material elements.
Realia must not be confused with terminology: the latter is primarily used in the scientific literature to designate things that pertain to the scientific sphere, and usually only appears in other kinds of texts to serve a very specific stylistic purpose.
Read this ELU question for more info.
If this is about what goes on in your mind instead of what's in the dictionary, the usual linguistic term is word finding difficulty.
Word finding difficulties are usually studied in the context of language attrition, which also includes first language attrition or FLA. In this context, you can also come across the term lexical first language attrition. (See for example, Lexical first language attrition (PDF) by Monika S Schmid and Scott Jarvis.
Word finding difficulties in one's native language have also been studied in the context of aphasia.
If the difficulty you are describing is not cognitive but about what's in the dictionary, realia (as suggested by Quill) may be the word you are looking for. (Note, however, that realia in the context of (language) teaching has a very different meaning.)