Your sleep/Schlaf example is very comprehensible - and a very clever way to think about how to structure your information! 
If you are worried about such side effects, then just call your frontmatter value “sleep” - why not? Or, if you prefer to use German, choose a less frequently used word like for example “Pennen” or whatever. 
I haven’t experienced such side effects so far - but that is mainly because I don’t search for text very often (and if so, I am searching for quite special words). I usually wouldn’t search for “Schlaf”, for example, as probably I would have tagged all notes about “Schlaf” with #schlaf (I am working a lot with tags - if you want to read something about this, have a look at this quite antiquated post…).