Version: 184.108.40.206 Writer
Is this question linked to regulatory requirements to have Welsh versions of things? Do you work for a Welsh govermental agency? Why not sponsor some development of LibreOffice to get these things done. From the link in the answer, there is a spellchecker dictionary but not other forms of Welsh localization.
For details of supported languages please have a look at: Language support of LibreOffice - The Document Foundation Wiki