Russian does use 'Nemets' to mean German or Germanic, and the verb _nemét'_ does mean "to be speechless." Bulgarian, on the other hand, uses /njam/ for 'dumb' but either 'germanski' or 'nemski.'

I'd question whether 'Slavic' comes from slóvo - 'word.' The Russian word for Slav is 'slavianín/slaviánka' (masc./fem.) and 'slaviánskii' for Slavic. The more likely root in my book would be 'sláva' - fame or glory.
Just my dve kopecka.

[oh, and to address Faldage's inquiry about my nom de guerre: slovo - word and voi - (thing)... Think in terms of Russian folk tales about domovoi, dverovoi, dvorovoi, etc. {The house/door/courtyard "thing" that watched over the home and could be source of either solace or mischief, depending on whether one paid the proper respect.}]