Call me crazy, but I swear I see a resemblence between the German word for 5: take fünf; morph the /f/s into /p/s as occasionally happens (cf. Welsh ceffyl vs. Irish capall, both cognates meaning "horse"); replace the /ü/ with /i/ (vowels foreign to one language are often merged with familiar values); and the /n/ becomes an /m/ because /np/ is always a tricky cluster and often gets slurred into /m/; (cf. the prefix in- becoming im- before words that start with m or p).
But all of the other numbers seem either Latinate (un, dau, tri, maybe naw) or unrelated to anything I'm familiar with (pedwar? chwech?).
Yes, pump and fünf are related. pump is also related to Latin quinque and thus also French cinq etc.
Welsh is P-Celtic, meaning that Proto-Indo-European "kw-" turned into "p-" (hence the many question words with p- where Latin has qu- and English has wh- and German w-). Irish is Q-Celtic, on the other hand, so it has c- (earlier q-, I believe) in such places.
So the word that gave Latin quinque and Greek pente (penkwe or something like that) gave something like quenque in early Celtic, then pempe and then pimpe in early Brythonic, then finally pump in modern Welsh. (pymp in Cornish. Always a source of titters for schoolboys. Along with poos for "heavy" or hwor for "sister".)
pedwar is just tetra/quattuor (as in quatro-) in disguise, from PIE kwetwor or so. And chwech is related to Latin sex, from PIE sweks or so.
Celtic is more closely related to Romance (and thus Latin) than to Germanic, so it's not surprising that some of the words still look similar - though some relationships are rather obscured by sound changes.
(pymp in Cornish. Always a source of titters for schoolboys. Along with poos for "heavy" or hwor for "sister".)
I'm not gonna lie, I did giggle at that. Especially if you have five sisters!
Yma pymp hwor dhymm!
Even worse if you use the "traditional graphs" variant of the Standard Written Form which is also accepted... "pymp whor".
It is in Cornish, yes. Or the word for "weight".
Related to Welsh pwys "weight"; all from Latin pēnsum.
Presumably wyth is related to French huit as well. And deg to Latin decem.
Indeed - wyth is related to huit/ocho/octo/eight/acht/... (all from PIE *oktō(u)) and deg to decem/deka/ten/zehn/... (all from PIE *dekm).
I know a little and I know where to look up more :)
The Geiriadur Prifysgol Cymru is good for etymology of Welsh words, EtymOnline for that of English words. Both also often list cognates in other languages such as Latin.