Is this normally pronounced as in English (i.e. as if it were spelt “háudrojen” in Welsh) or according to spelling? The TSS pronounced bioleg sort of “mixed” as “baioleg”, although in wiktionary the pronunciation is said to be according to spelling. Or are there multiple possibilities?
When I was teaching Science through the medium of Welsh we generally either pronounced it in the English way or in a more Welsh way as 'Heydrojen'.
Some other elements have specific Welsh spellings:- eg Sylffwr, Mercwri, Ocsigen