@Gargron
And it's not just expressions and turns of phrase, which are unique not only to each language but easily to each region.
It's words. There is often no exact match. Many words in one language are not a precise correspondence to those in another.
So an elegant turn of phrase in one becomes a wordy explanation in the other. Or a misinterpretation.
Only someone fluent in both languages can convey the true meaning accurately and eloquently.