can be up to 3 months from resume submission to offer, with 4 or more interviews in office or via phone/video. Can be shorter for internships with a view to being permanent.
Simple greeting to the interviewer, shows knowledge of the culture and willingness to learn and use Japanese
It gets better with practice and can be a good summary of who you are. Simple and short is fine.
Concluding the Japanese part of the interview or leaving the interview, it is always good to say thanks.
If you know keigo great, but not necessary to worry about it during the interview. Japanese level is evaluated but even if you don’t know keigo, if you have a good command of business Japanese, or just are able to express yourself in standard Japanese, that is fine. Language ability is a threshold but keigo is very rarely the determining factor for a foreign hire.
Using your Japanese
I believe it is good to include Japanese in the interview, even if you are N3 or N4 (JLPT levels).
If you know other languages it is a plus if they have a use within the hiring company
Language of the home country - English, a European or Asian language, and/or any language in which the company conducts business in its home area.
Asian language - If the Asia headquarters is outside of Japan, it can be useful or beneficial to have command of Chinese (Mandarin or Cantonese). Just a nice to have element on the resume unless you are specifically involved in supporting Chinese customers.
On the other hand, if it isn’t related to the company’s business, there is no need to include a language unless there is fluency unless it’s an ongoing hobby or interest you want to mention in the interview.