A requested clarification about the NWB language

Are there any classical sources which intimate that as of the 1rst c. BCE there were other linguistic groups in the area between the Atlantic Ocean French coast (roughly north of the Loire just for purposes of the query), and in today's Belgium, Netherlands, Denmark and Southern Scandinavia, Germany, Poland (roughly to the Vistula , again f.p.o.t.q.) which were neither Celtic nor Germanic? The only source that comes to mind is Caesar's mysterious "Belgic" (which no other source repeats?)... If Latin sources have no notion of a distinct NWB language, considering this a "Germanic" area (basically) then must we not assume that NWB was "Germanized" before the 1rst c. BCE? When?