I have been made aware that there is now a Welsh spellcheck dictionary that we should consider incorporating. Currently there is no Welsh spellcheck support to my knowledge.

http://techiaith.cymru/hunspell/

The author reports it should be good quality:

The quality of our Hunspell spellchecking dictionary is very high as it is derived from Bangor University’s  commercial proofreading software, Cysill, and therefore covers the vast majority of Welsh forms, including conjugated verbs and prepositions, plural adjectives and mutated forms. Cysill is used extensively here in Wales, and the free online 3000 character version of Cysill at http://www.cysgliad.com/cysill/arlein/?locale=en ) can give you an idea of our dictionaries coverage of Welsh forms – just run any Welsh text (such as “Mae heen gwlad fy tadau”) through it. Do note, however,  that our Hunspell dictionary does not include the sophisticated grammar checking features found in Cysill.
 
We believe the typical Welsh user would be very happy to run their web input through our Hunspell dictionary. The possible issues would be that nearly every Welsh speaker is bilingual Welsh/English, and would probably want to be able to use both languages at the same time. However, I believe Chrome now features multilingual spellchecking that can enable a number of languages simultaneously. Other issues are that some very informal forms (mainly dialectal slang) and derogative words are not included. Some of the latest neologisms are also missing where the term has yet to be standardized in the written language.