How do master’s degrees work in Japan + is it worth it?

So now I'm spending a few months in Japan as an exchange student and I was thinking about the possibility of coming back for the master's degree. I have a JLPT N1 but I'm still not very confident in my skills (although I expect them to get better during these months) + my field is linguistics so I should probably become very fluent to do a master's degree here. Also I'd like to work in a university but of course I still haven't decided whether that would be in Japan or not.

So these are some questions I have, please reply to whatever you have information about:

1. Are Japanese master's degrees as valid as European titles in Europe? Could I access a European PhD with it?

2. If you do a master's degree (but not as part of a PhD) and then you decide to do the PhD in Japan, is it only 3 more years or 5?

3. How difficult is it to get in?

4. Are there many master's degrees in English, in case I feel like the Japanese one is too much for me?

5. Do all of them start in April or can you find some that start on September? I've read they exist but I don't know if they're common.

6. Overall, do you have any other recommendation/warning, or any reason to say that it is or it is not worth it?

Thanks a lot.