Here are some examples of how "beautiful" is expressed in different Celtic languages:
* Irish (Gaeilge): *álainn* (pronounced "ah-lan")
* Scottish Gaelic (Gàidhlig): *brèagha* (pronounced "bre-ha")
* Welsh (Cymraeg): *prydferth* (pronounced "prith-verth")
* Breton (Brezhoneg): *kaer* (pronounced "kayr")
* Manx (Gaelg): *beayn* (pronounced "bayn")
The word you choose will depend on which Celtic language you're interested in.