That makes sense, but seemingly more from a perspective of "I'm going to still have just a single CSS file." Talk19Zehn didn't appear to explicitly specify where they were intending to store the CSS, so this is a good point to discuss regardless.
By Talk19Zehn's reference to the previous suggestion, I was expecting the intention was to leverage the language-specific CSS file like phpBB's prosilver already incorporates (./theme/en/stylesheet.css, ./theme/de/stylesheet.css, ./theme/fr/stylesheet.css, etc.).
Such that "translation contributions" are then the submission of additional language subdirectories for the style, versus having to edit in the additional language translations in-line within a central file. This was part of the attempt to mimic "best we can do" in absence of having an actual phpBB language directory supported in the style; where a common.php in a language-specific directory would provide additional style-specific language variables, and making "contributed" language submissions no different than how they're handled for extensions and phpBB itself.
Which would mean "keeping things simple" is that every language targets exactly the same class name, rather than a language-dependent class name. What will cause one language versus another to be displayed will be the fact that overall_header.html imported ./theme/de/stylesheet.css instead of ./theme/fr/stylesheet.css, etc.
The idea of language-specific class names is a great solution though, if the intention was to have only a single CSS style sheet defining all languages.
Edit: Ah, I see now Talk19Zehn's edit of their previous post, and now specifically stating that separate per-language CSS files are in fact being used. (artsmonode.css, versus artsmonoit.css, etc.) So separate CSS files instead of a single CSS file, but not using the ./theme/<lang>/stylesheet.css arrangement like prosilver had initiated.