Felgen used the pseudonym "Jean Nicolas" when translating the tunes.
As to how this came about, in 1964, a producer from EMI flew Felgen to Paris to meet with the Beatles, who had agreed to make two German recordings, as it was common for some artists to release music for the German market in the 1960s. However, they had rejected the original translations and refused to sing them, boycotting the radio session.
Felgen was given less than 24 hours not only to translate and finalise the lyrics in a manner which the group would find acceptable, but also to coach the Fab Four to sing phonetically in German.
However, the Beatles hated the idea of making foreign-language recordings - thus, those two songs recorded in Paris, assisted by Felgen, were the only songs the Beatles ever recorded in German (although two unofficial recordings were released eventually). They were also the only songs the group ever recorded outside of London. Unsurprisingly, the Beatles found fame in Germany with their own music in English.