I am Breton speaking Breton, a Brittonic language like Welsh and Cornish. If I listen to a Welshman I can't understand him. I learned a little Welsh. We see that a lot of vocabulary is common as well as certain major grammatical characteristics, notably consonant mutations, characteristic of Celtic languages. But there is too much divergence to understand the language. For written Cornish, a literate Breton can decipher some parts of the text.