Breton and Welsh are not mutually intelligible at all if one of the speakers hasn't learnt the language of the other. Words seem alike: names of trees, animals etc. Most verbes are very different, syntax is very different, pronounciation is different...

Is Breton similar to Welsh?

As one of the Brittonic languages, Breton is related closely to Cornish and more distantly to Welsh, while the Gallo language is one of the Romance langues d'oïl. Currently, most Bretons' native language is standard French. ... Ethnically, along with the Cornish and Welsh, the Bretons are Celtic Britons.

Is Breton a Celtic language?

Breton language, Breton Brezhoneg, one of the six extant Celtic languages (the others being Cornish, Welsh, Irish, Scottish Gaelic, and Manx). Breton is spoken in Brittany in northwestern France.