"A red hat"
In for example Spanish and Polish, the adjectives change their ending to suit the gender of the noun. It's very interesting that Welsh changes the start of the adjectives to suit the noun, does anyone know if this technique is used in other Celtic languages like Breton or Irish?
It's not so much adjectives inflecting for the person like it is in French and German (and I assume Spanish and Polish.) This is just the same sort of initial consonant mutation as you find in other situations in Welsh. Also to answer your question yes, all the celtic languages have initial consonant mutation, though I can't comment on when they occur in the Goidelic branch, though I do know that Cornish also mutates adjectives after feminine nouns e.g "An gath dha" which is "Y gath dda" in Welsh (Dha and Dda are pronounced the same) and mean "The good cat" in English. Since both Welsh and Cornish mutate after feminine nouns we can pretty safetly assume that Breton does too.