I have to agree that if you're already fluent/ almost fluent in a language it does seem a little pointless to study it to degree level - yes, there will be plenty of other modules on history/ literature/ culture/ politics etc. too but you could say the main "employment feature" (not sure if there's a better phrase for that - basically the key thing you learn that will actually be helpful in getting a job) would be the language skills.
Computer Science is a very different degree of course, typically with a VERY high maths content. If maths isn't your strong point you may find it very challenging - despite common misconception, the main content of a computer science degree is maths and theory about computing, not actual coding. The high maths content is also true of many other STEM degrees, e.g. most physical sciences.
Have you considered other degrees in things like international relations, politics, international business, etc.? If you wish to pursue your interest in Japan you could look into not only degrees with a year abroad (btw, usually no guarantee you get the university or even country you originally request, depending on the uni - make sure you check this out) but also consider doing your whole degree in Japan?
If you could tell us your A level subjects we could give more suggestions on what degrees would be suitable
In terms of taking a gap year or not, I personally would say you could give UCAS Extra a shot and see what you can get, but if you're not 100% happy with the results from UCAS Extra (e.g. if no universities you like are offering the course you want) then don't be afraid to take a gap year and reapply. A gap year could be a great opportunity to travel, earn some money, study a useful subject, perhaps go to Japan, etc.