We all would be happy to know that, but as long as we don't have any literal testimonies, we cannot tell the Celts from the Germanics by their language. It is a hypothetic distinction with no reference to historic reality, unfortunately.

On a map you find tribes named Tungri, Treveri, Vangiones, Mattiaci and Nemetes who cannot definitely be marked as Celtic or Germanic until today.

On the other hand, in wide areas of the Germania like today's Hesse (between today's Frankfurt and Kassel) Celtic oppida were placed even in Iron Age times. Only few generations before the Roman empire expanded this far north, Celtic and Germanic lands were apparently embracing each other on both banks of the Rhine.

Probably the differences grow with the geographical distance, but along the Rhine, both cultures seem to have blended smoothly. I suppose even their languages might have approached each other. Vortigern mentioned the differences were not too big.

The original distinction of Germanic and Germania was a geographical line, marked by the Rhine. Most likely Caesar defined it to make sure he conquered Gaule wholly, as he made sure that Germania beyond the Rhine was nothing to yearn for (and Tacitus later affirmed his descriptions of a bleak landscape).