I've studied Japanese for about 4 years now, and next year will be the first with a teacher (hopefully). Besides Japanese I'll have German too, which I've studied for 3 years. Do you think that having Mandarin on top of those will be too difficult? (By the way, it's at secondary high school.)

Welcome at the forum.
The fact that Japanese uses chinese characters will help you not to mess around too long with chinese writing. Chinese is grammatically simple so I think if you don't plan to get fluent speeking and just learn how to read chinese texts (even if it's with an dictionary) you can achieve this while learning German and Japanese. In never learned Mandarin, just started learning Japanese and learned German as my native language, so I don't really know much about the difficulties of learning these languages, but if you take your time I'm sure it's possible to leanr all three languages at the same time.


Edit: There's a typo in your other thread こんにチワワ should be こんにちわ.
If you want to express that the wa is spoken long like the y in heyyy, I think you should use something like こんにちわ～ or こんにちわぁ or something like this, but not really sure what way of writing this is the usual way in Japanese.

About こんにチワワ, it's a joke. It's like konnichihuahua. Plus, it's supposed to end in ha, not wa. Or, you could say wa, but that makes the sentence more feminine, or am I wrong?

I'm just afraid that I'll mix up the characters, since Chinese characters don't have the same meaning as the Japanese ones.

Ah you're right. Of course it has to end in は and not わ. Seems like I wasn't paying attention properly.
Yes the different meenings coul be a problem. And another problem could be, that you use japanese simplifications in chinese texts and vice versa.
But just try it. I think you'll realize when it's too hard.