but there I have only one custom.css, but so far I had to have 2 templates, for EN and DE languages
Hi. You can add a page class in the menu item settings or classes for sections in the Layout Builder. And use these classes as parental to create an isolated code.
For greater convenience, you can use custom.css as a master file. Put in the same folder any css files for example custom-en.css, custom-de.css and import them to custom.css.
Thus, in custom.css, you will have only this code.
@import "custom-en.css";
@import "custom-de.css";
All the rest code transfer to the appropriate files.
If you are using HU 2 there now, it is possible to create a custom folder and place any number of files there. But it is not very convenient. I prefer the method described above.