I have dabbled in translating Brassens to English and was chatting about this with a French friend of mine who translates Jake Thackray into French. There are a number of problems, some perhaps specific to Brassens.

Firstly, he often deliberately uses archaic French. Secondly his lyrics are frequently strung out from start to last in the subjunctive which is not without its difficulties in English. Thirdly he frequently rhymes on the closing "...re". Fourthly some of the songs either have specific French references which don't resonate in English and fifthly some just fall flat in English.

Some are pleasingly filthy, though, including the excellent "Melanie" about a parishioner who pleasures herself with church candles - but only the best beeswax "because she was a good catholic".