Sorry I had to re read, they probably will if they didn’t forget it.

Is it normal to say they in English if you refer to a single person?

Yes it is normal, I am just very autistic.

Depends on the situation.

Are you working steady with this colleague?

Do you guys sometime talk?, if both no than you don't have to send him and he'll probably won't notice.

It's a she, I had a crush on her last year, so giving her birthday wishes on WhatsApp was part of me trying to get more intimate with her. We are working continuously together and I'm closer to her than to other colleagues. But since then I gave up on it and wanted to keep more distance, so I wasn't sure whether I should do it this year.

Saying happy birthday on WhatsApp is ok, at least in here it seems like a non personal thing to do, so she might receive many messages on her Birthday(though I am not sure what is the proper conduct where you are from so you know best)