What's the English name of the oi sound written as "eu" and commonly found in Germanic words like Deutschland, and names like Euler and von Neumann?
There's no single word for it. It’s the diphthong /ɔɪ/, a glide which ‘begins between back half-open and open positions, moves upwards and forwards towards [ɪ]; lips open rounded changing to neutral’ (‘Cambridge Encyclopedia of the English Language’).
The oi sound is a phonemic diphthong, one of the three main German diphthongs.
It is a combination of the "open mid-back rounded vowel", like in "dog" in American English, and the "near-close near-front rounded vowel", which doesn't exist in American English.
The referenced links below have audio clips to hear the vowels.