Because I‘m German and that‘s the way we use them in Germany. So my German mobile keyboard does this automatically, yes. Oftentimes I change it in English messages, sometimes it slips.

I'm also German and have never seen such weird quotes. Maybe this is some weird windows charset issue but definitely not widespread way of quoting text.

It definitely exists in German as well as some other European countries, https://en.m.wikipedia.org/wiki/Quotation_mark I checked a few German newspapers, some use it, some don’t

I don't know why, but in my browser, the closing quote is showed correctly (forward slanting) in the edit box, but backwards slanting when submitted. Weird.

„how“

Different fonts, most likely (I can't say for sure because I customized the CSS for HN using the Stylus browser extension)

I'm guessing they're using a mobile device with a keyboard that does this automatically.

Wait until you see how the French do quotes

Mandarin Chinese keyboards「have entered the chat」.

Do Chinese people actually do that? I thought 「」 has ever so slightly different bearing than "" but then again I don't speak Chinese

Sometimes. It's formal, but text autocomplete systems can sometimes insert them. The other form of quotes is similar to English quotes, but looks a bit different.