Another one I've noticed is using "I've" as a contraction in e.g. "I've a meeting to attend". Seems totally reasonable but for some reason native speakers just don't use it that way.

I’ve is only used when there is a verb to follow and the have is part of the verb’s construction.

As in “I’ve done it” or “I’ve seen it”

It would not be used before a noun, in the context of ownership, as in “I have a meeting”

Wait, what? Englishman in my 50s here and I use phrases like that all the time — “I’ll be missing standup cos I’ve a GP appointment”, “leaving at lunchtime as I’ve a train to catch”, “gotta dash, I’ve chores to do”. No one’s ever said I sound German!

I think it's more fair to call it a distinguisher of American English vs. British English.

Even just reading "I've a train to catch" gives a British accent in my mind.

A particular part of Britain as well. I have never used “I’ve” in that way ( I speak more RP than with an accent)

Nah that’s just Americans. Brits and Aussies say it all the time. Not sure about Canadians.

Could also be French speakers. They would say "J'utilise le format .avif depuis quelques années." I think the "depuis" throws off the French speakers when they translate that literally as "since some years" instead of "for some years".

Another common tell: I wake up in the morning in the US/Pacific time zone, and see the European writers on HN using "I have ran" instead of "I have run".

German speakers usually have very good English, but this is already one of their tells.

Fixed that for you.