The Germanic tribes WERE Celtic tribes. Sorry, but this is pretty basic stuff of history.
>"Viking" is not a term for Scandinavians generally, but for men who went about plundering and marauding.
Not one historian in a thousand would identify the word "Viking" with other than Scandinavians in general, and Norse in particular.
Linguistically, Celtic is a term for a branch of the Indo-European family of languages, which includes the extinct languages of the ancient Gauls and Galatians, recently dead languages like Cornish and Manx, and living languages like Irish Gaelic, Scottish Gaelic, Welsh, and Breton. It is separate from the Germanic branch which includes German, Dutch, Frisian, English, Norwegian, Danish, Swedish, Icelandic, Faroese, and Afrikaans, along with dead languages like Gothic and Old Norse.
In ancient times the Celts are mentioned in various Greek and Latin writings under various names, in Gaul (France), northern Italy (Cisalpine Gaul), Britain, Ireland, and occasionally further afield. The Galatians were Celts who settled in Asia Minor (although the Galatians St. Paul wrote to were Greek-speaking--not everyone in "Galatia" was Celtic-speaking). One group of Celts sacked Rome in or about 387 B.C. The Germans were mostly beyond the Rhine. The Romans first encountered them when one German tribe, the Cimbri, began wandering and defeated a Roman army in 113 B.C. Some of the Germans were later subdued by the Romans but most of them remained free (thanks in part to Arminius' victory at the battle of the Teutoburger Forest in A.D. 9). There may be a few cases where it is unclear if a certain tribe is Celtic or Germanic (or they may have been mixed) but for the most part the Celts and Germans were distinct groups.