English will get you by in Berlin. If the startup has international employees, then English will be spoken heavily inside the startup (usually).

Outside of your startup, you'll want to at least learn enough German to be able to ask for things in a shop, order at restaurants. Though even if you were to speak in English, you'll find most people will be able to respond in English and help you out, but that depends on how far you travel. Once you start getting out beyond the ring (metro system), you'll find the level of English degrade.

One tip when you start out here: Genuinely apologise to someone that you do not speak German in German, ask if they speak English in German, and they will almost always be happy to speak in English. Start blurting out loudly in an American-English accent and you may not receive the same level of hospitality.

Source: Live and worked for a few startups here in Berlin, speaking from anecdotal evidence based on experiences from both German and expat friends

Standard in university groups is English if necessary (i.e. at least one person doesn’t speak German) and German otherwise. I would expect it to be roughly the same for highly technical startups.