I'm going to live in Japan with my future wife. She is L1 (first language) korean speaker but lived in Japan for over 10 years making Japanese her L2 (second language). My own L1 language is German and my L2 language is English.
Now as I'm going to be living in Japan I want/need to learn Japanese. But I'm also very interested in learning Korean. She on her side wants to learn my L1 and L2 languages by coincidence. It gets more complicated when we are going to have children. They most likely will be L1 Japanese speakers as they will grow up in Japan. English should be their L2 language. Yet I suppose it would be nice if they spoke their parents L1 languages to a degree for visiting each others families and such. I put up a table, stars show importance.
Wife Husband Children L1 - korean L1 - german L1 - japanese **** L2 - japanese * L2 - english * L2 - (english) *** L3 - english *** L3 - japanese *** L3 - (german) * L4 - german ** L4 - korean ** L4 - (korean) *
As you can see there is hardly any combination. I read that you shouldn't learn/speak more than two languages at the same time as learning quality is likely to decrease.
Now I'm wondering what the best approach to this situation would be. To achieve a fluent level I'm convinced a continuous usage is nesseccary. Yet how should that work without crossing the 2-at-a-time border. Probably switching on an everyday, weekly, monthly basis wouldn't be beneficial either, wouldn't it?
I also fear 4 languages will degrade the childrens language abilities. As I heard about that back in university in lectures. Although I'm not sure this is state of the research anymore.
Are there current studies or any reading matter about this?