Is it polite to call German foreign teachers by their first names? Besides, I sent her a message very late today. I feel a little uneasy about her, and I want to give it to her.

It is ok to say hello XXX directly, because foreigners feel that if you add Dr. and so on, you will feel alienated. If you say you're sorry, say you're sorry to bother you so late. But if you send her a message, don't apologize if people don't reply at all, because foreigners think I should only deal with my work during office hours, and people won't talk to you unless I am a very nice person.