I think using IRL language family trees would be a bit ridiculous. A fifteen year old can speak 30 or 40 languages? There's a reasonable genius polyglot and then there's an obvious thinker. And until we have a multi-continent spanning empire, it would be fucking superfluous. And by the time we had a multi-continent spanning empire we'd probably have a imperial language, rending that kind of numbers of languages moot, making having irl family language trees superfluous.
English and a handful of Asian languages are all he's going to need/want for probably the duration of the quest. German is definitely a flavor language that could come in handy, but multiple Asian languages will be super handy for spying and stuff.
So, will be going with what
SwiftRosenthal said about language trees.
German itself can be broken into several dialects. Bavarian baaaaaarely resembles high German.
'Ich weiss nicht' is 'I don't know' in high German (the national German used at the state house and stuff and from Hanover) 'I woas nett' is Bavarian for the same thing.
Hell even 'Bavarian' is wickedly different. Bayernische v Borisch.
Stein is pronounced with an 'i' sound, but near the dutch its pronounced with an 'e' sound. In Berlin German 'Ich' is pronounced 'Ik' instead of 'Ic-hhh.' Some places 'und' is pronounced 'unt' and those are the examples I learned when I took German in highschool. We didn't even get into the real meat of the differences in the regional dialects.
Yes, they were 'Honorary Aryans.'