It's not possible. Maybe cornish-breton because they were similar and with straight links, but not cornish-welsh or breton-welsh.
It's possible to understand welsh if' you'are good breton speaker, but it's easier for a welsh speaker to understand a breton speaker.
Welsh tipical sounds like "ll" pronounced like a wet l, and "rh", "ch"... are like strong sound parasites. Syntaxis, use of the verbs in welsh can be difficult for a breton speaker.
But basic words are similar :
bread / bara / bara
mountain / menez / mynydd
blue / glas / glas
low / izel / isel
Un exemple of a quite "similar" sentence :
The weather is beautiful
"Ta an amsair go brabh" (gaelic) (I ain'nt sure)
"Mae'n tywydd yn dda" (welsh)
"Brav eo an amzer" (breton.
2 similar words in gaelic and breton but diferents in welsh... but pronounciation makes it impossible to understand.
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