Many are, as most developers start their careers on F95&Co, but there are lots of people who just look for adult games via Patreon aswell, and those would be entirely left out if L&P were to release his project in his native language.
Or as Ginger put it earlier, a way to alienate the audience.
Even if, in theory, every patron had a way to get the fanmade translation, I'm pretty sure it would be worse than the original. You'd either end up with a google translation or people who simply aren't 100% fluent in both languages. People up for this task are usually not working for free as it's quite a bit of work, especially for a project such as AWAM where there's a ton of dialogue compared to other VN's. Either way, he'd likely have way less patrons if the game was released in German.
But on another note, I don't believe for a second that he's actually writing the script in German first and then write it down a second time in English. English isn't my first language either, but I'm not gonna write things down in my native language first before I translate it. That's straight up stupid and doesn't make any sense whatsoever. If I were to struggle formulating a sentence, I'd google it straight away instead of having to type everything down twice. And according to L&P, he's got his translation/proofreader guy anyway, which would make it even more stupid. lol