In the latest installment (the sixth), the translator justifies leaving the two-line refrain of a famous poem untranslated, by saying that it's hard, and others have done it badly, and anyway, it contains English cognates that non-French-speakers should be able to figure out for themselves.
Hmmm. How do others feel about that?
I think it is justified and kept within reasonable bounds of "poetic license".
It doesn't sound like she is being reckless or arrogant, rather than admitting a weakness: "I tried my best...but, in one instance, I gave up", "the famous refrain...defeated me." , "I could find no English words...", "please forgive me..."
It is not uncommon to use foreign tongues (especially Latin and French) in English artistic writing, as titles, epigraphs, stylistic flourishes etc. Even though this is a translation, the artistic effect is along the same lines.
If you don't know what something means, thousands of dictionaries and grammars are only a click away to help. Unless one confines oneself to the most understood words, now and then (s)he is likely to make people look up English words in the dictionary too. Introducing people to different words/expressions - including foreign ones - is usually not a bad thing.