What is the primary source for this quote by Julius Caesar's on Celts and Germans?

All nations/races "wax and wane", have their ebb and flow. The Celtic people were the dominant power in Europe for 1500 years before the emergence of the Roman empire and the huge population explosion out of Scandinavia.

We know that the Celts were politically and militarily dominant over the Germans for centuries prior because the names in the German language for "King, Law and Iron" are all originally Celtic words

Also, Germany unlike Gaul had no major cities but rather lived in huts in their vast forests, which made them much harder to attack and conquer. Gaul had major cities all over the 3 major Gallic provinces...Caesar merely had to focus his military attention there and thats what he did

Performing first a search for all occurrences of "German" and then of "civiliz" in The Gallic Wars suggests that the most relevant passage is from Chapter 24 from Book 6 (my emphasis):

Chapter 24

And there was formerly a time when the Gauls excelled the Germans in prowess, and waged war on them offensively, and, on account of the great number of their people and the insufficiency of their land, sent colonies over the Rhine. Accordingly, the Volcae Tectosages, seized on those parts of Germany which are the most fruitful [and lie] around the Hercynian forest, (which, I perceive, was known by report to Eratosthenes and some other Greeks, and which they call Orcynia), and settled there. Which nation to this time retains its position in those settlements, and has a very high character for justice and military merit; now also they continue in the same scarcity, indigence, hardihood, as the Germans, and use the same food and dress; but their proximity to the Province and knowledge of commodities from countries beyond the sea supplies to the Gauls many things tending to luxury as well as civilization. Accustomed by degrees to be overmatched and worsted in many engagements, they do not even compare themselves to the Germans in prowess.

As you can note from the passage, Caesar remarks that the Gauls settled near Provence (The Province) have become softer through that association and proximity, and are no longer capable of besting the Germans whom they formerly were able to defeat routinely. However I could find nothing near to any occurrence of either search string (in this translation) that suggests that Caesar desired to conquer, or recommended conquering, the Germanic tribes.

I would suggest reading Book 1 of The Gallic Wars (link has both English and Latin if you want to see the untranslated text), which is as much a political history of the conquest of Gaul as it is a military history. Julius Caesar starts the book with a description of the political landscape at the time:

All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in our Gauls, the third. All these differ from each other in language, customs and laws. The river Garonne separates the Gauls from the Aquitani; the Marne and the Seine separate them from the Belgae. Of all these, the Belgae are the bravest, because they are furthest from the civilization and refinement of [our] Province, and merchants least frequently resort to them, and import those things which tend to effeminate the mind; and they are the nearest to the Germans, who dwell beyond the Rhine, with whom they are continually waging war;

The last section is typical of the attitude of the Romans (and very similar to colonizing powers throughout history), as it portrays Rome itself as the source of civilization. Simply put, the closer to the influence of Roman culture, the more civilized the population. By further comparison to the comparative cultural merits of the Gauls and Germans, he notes in chapter 31 (my emphasis):

But a worse thing had befallen the victorious Sequani than the vanquished Aedui, for Ariovistus the king of the Germans, had settled in their territories, and had seized upon a third of their land, which was the best in the whole of Gaul, and was now ordering them to depart from another third part, because a few months previously 24,000 men of the Harudes had come to him, for whom room and settlements must be provided. The consequence would be, that in a few years they would all be driven from the territories of Gaul, and all the Germans would cross the Rhine; for neither must the land of Gaul be compared with the land of the Germans, nor must the habit of living of the latter be put on a level with that of the former.

Although it is hard to pick out specific passages, the threat posed by the Germans was that they were much more aggressive. The danger to Rome from the Germans was not only in the lack of the civilizing influence of the Rome, but in the perception that they could not be contained to Germania if they succeeded in overrunning Gaul. From chapter 33, again with my emphasis:

That, moreover, the Germans should by degrees become accustomed to cross the Rhine, and that a great body of them should come into Gaul, he saw [would be] dangerous to the Roman people, and judged, that wild and savage men would not be likely to restrain themselves, after they had possessed themselves of all Gaul, from going forth into the province and thence marching into Italy (as the Cimbri and Teutones had done before them), particularly as the Rhone [was the sole barrier that] separated the Sequani from our province. Against which events he thought he ought to provide as speedily as possible.

The Wikipedia page basically summarizes the entire book into two sentences. It's worth the read and short enough not to be much of a time commitment.