There are "official" dialects and then there are those that just pronounce the words differently or uses different phrase styles.
As for a language dialect, say using different words, like the various non-English languages spoken in the UK, or ones that are based in English variants from 500 year ago and never really changed much over the years, these may need to be part of some install of LO.
Look at all the different language dialects that India has. Only a few are part of the "official" list of languages that LO supports.
Regional languages or dialects could be a real "mess" to try and get LO to support all of them.
How many languages/dialects are supported right now? 100, 150?
How many thousands are out there that may never be supported due to the small market? I could only guess.
I lady I met was overjoyed when she heard that OOo [before LO ever came out] supported Hebrew. She worked in a tourist center that needed to use her English, French, and Hebrew skills to answer questions and create documents.
If you want to create a dialect of a language, that is close to the "root" language that LO supports, you could create a dictionary that uses the "root" language and add the dialect elements. Look at Spanish. There are about 20 regional dialects that have come out in dictionaries for LO based on the country that it is spoken in. It is still Spanish, but with a regional name included as part of the dictionary's name. Spanish - Mexico, Spanish - Columbia, etc..
The hardest part of the whole process it finding the needed documentation to create the .aff and .dic files. When I last searched, I came up empty. So I just used some common sense and used the "default" en_US .aff file and then just used a word list of all the words with their various prefixes and suffics included. I ended up with over 700,000 of them. I did have a en_US dictionary that included over 2 million words and their proper variations, but that seemed to go too far. Some of those really "rare spellings" might be not so popular with certain English professors or bosses, even though they were correctly spelled.
As for the UK, I saw a person's list of "proper" Shakespearian era words. Someone told me that in that era, they spelled their words whichever way they felt like. So I decided not to make a dictionary for that.
But there are other dialects that could be added. All it takes is some willing souls to make it a working project.