No idea where you're getting it from, Germans are Nemci in Russian as well. It's rather "unable to speak the language", meant for all foreigners but later stuck to Germans, presumably because German traders were the most common foreigners.
Nem/нем literally means "mute" in Serbian, perhaps it's a latter evolution per region either way.
If making one mistake discredits the entire take, I'd hate to be your conversing partner ever.
You are not my conversing partner. You are simply someone who emits nonsense onto the internet and people should be aware of you willingness to confabulate reality.
Very far from Serbian only. Bulgarian, Russian, and even Balti-Slavic like Latvian is similar enough.
Same in Russian
нем\немой - mute
немота - muteness
But yes, we do use Germany for country's name :)
I wanted to check; are you implying that Russian is not a Slavic language?