For the record, in English, quotation marks are always above letters: 'like this' or "like this" (or, ‘like thisʼ and “like this”, to use the typographically more correct glyphs, although HN renders all of these pretty badly with Verdana).
Single or double is a matter of style—British English style guides tend to prefer 'single quotation marks', and North American English style guides prefer "double quotation marks". But alternate between the two for nested quotes (or avoid nesting to improve clarity):
Joe said 'Jen said "yes!" when I asked her to marry me!'
„This“ style is strictly German, as far as I can see.
„hi“ (German; ends with U+201C LEFT DOUBLE QUOTATION MARK)
„hi” (Polish; ends with U+201D RIGHT DOUBLE QUOTATION MARK)
"Hi $COLLEAGUE. I'm having a problem with $TASK: I'm trying to accomplish $GOAL but I keep seeing $ERROR. I've tried $WORKAROUNDS, but none of them have fixed the problem. I think it will be fastest if I can demo this for you; do you have 10 minutes so I can walk you through it interactively?"
I’ve tried to suggest to some of them that they lead with their request, but that has not helped much.
When I come back after lunch and find a message saying simply „hi“ from 30 minutes ago and now that person is AFK it spreads out the conversation over the whole day.
I am not saying you should drop the formalities, but there is nothing that forces you to split it up into multiple messages. Just write „Hi“ followed by the question in the same message.