"[Forum posts made by DDTM in English] just proves that he is capable but simply refuses [to speak English in DMs]." - Hober Mallow
He simply refuses, just like you were/are capable of speaking German to him, and simply refuse. You have your reason, and so does he. Your reason is foolish inflated nonsense, and his we can't evaluate because he's not participating in this discussion.
At the time you took your ill-advised affront at his polite request for you to speak German with him, you believed his reasons were that he had a language barrier. yet you insisted on speaking English because 'German's an OOC language, but English isn't'. You clearly valued your 'immersion' higher than accommodating someone's language barrier. That clearly makes you the one acting inappropriately, regardless of whether you still think you're correct that English = Galactic Basic and German = some language that doesn't belong in Star Wars.
This subsequent "proof" he can speak English does nothing to bolster your initial selfish and inconsiderate position at refusing to accommodate someone else because you can't spare that tiny bit of enjoyment from your mind-bogglingly fastidious definition of 'immersion'. You weren't entitled to be High and Mighty then when you thought he couldn't speak English well, and you're not entitled now. You also don't get to retcon your reasons after the fact: Nowhere previously have you represented the reason you didn't want to speak in German was because you can't speak it well and you're worried about nuances in negotiation. That's total nonsense you've invented Now. At the time, you got puffed up and ruffled because you didn't want to break your precious immersion, period, end of stupid story.
Get over yourself, and stop feeling entitled to high standards of fun at the expense of someone else. Nobody is going to be forced to speak any language they don't want to in DMs. There's consequences for your choices and his choices (you don't 'get' to negotiate with this guy for the release of your NPCs, and he doesn't get to RP with you e.g.), but there's no 'forcing' going on. You made a choice, and have created this thread to whine that your choice should be validated. Some people validate it, and some of us think you're full of it.
@Jevon: No, English is not Galactic Basic, in SWC or otherwise. English is an OOC language that stands in for whatever language the player wants their characters to speak. I describe Kay's speech in English, but he isn't speaking Basic. I don't speak any other languages, and there's certainly no established Verpine Language for me to write in. OP is making an illogical leap that English = Galactic Basic (upon which he bases the conlusion: therefore German is not, which leads to his 'damage' at German breaking his immersion). I've already gone over why it makes no sense to designate (arbitrarily) an OOC language as equivalent to an IC one. SWC designates English as its official language so there is a clear expectation of what language the game world will use to describe itself, and what language will be used publicly by the player base. It doesn't mean English is Galactic Basic.