The Greek that is translated as “The spirit is willing, but the flesh is weak” or similar in English is translated in the German New Testament translation by Berger / Nord (publ. 1999) with Der Heilige Geist macht mutig, aber als bloße Menschen sind wir feige or “The Holy Spirit makes us courageous, but as mere humans we are cowards.”

The now commonly-used English idiom “The spirit is willing, but the flesh is weak” (meaning a person pretending to be harmless) was first coined in 1526 in the English New Testament translation of William Tyndale. (Source: Crystal 2010, p. 281)

For other idioms in English that were coined by Bible translation, see here.