Lots of things are possible, fewer are likely, even fewer attested, even fewer attested without doubt.

Though it’d have a special irony to it: Some Roman asking a bunch of probably Austro-Bavarian or Alemanic warriors “who are you” and getting the answer “Germannen” – “spear men”. Were Italy an Island they would’ve said “sailors”.

Probably more likely than Romans observing those tribes, noting more or less bog-standard javelins, not exactly a rare or particularly notable technology, and saying “hey let’s ask them what they call javelins and add a -man after that for no apparent reason”. Even if they specialised their tactics etc. around those spears would you call Greeks “Phalanx people”?