Hi, I guess you're Korean based on your name. Kagi is used by people all over the world, and I believe the team usually listens to native speakers and local people when it comes to their languages. It would help if you could provide some supporting points for your ideas or suggestions.
As for the Jeju dialect, Kagi Translate relies on AI (LLMs), so support for the dialect may not be fully reliable. Some other Korean users and I have previously raised concerns about Korean translation, because even Standard Korean (표준어, pyojuneo) was sometimes mixed with other languages, such as Chinese, in some Kagi services. Jeju dialect is likely to be even harder to handle, since it has much less training data and far fewer standardized resources than Standard Korean.
You can still enter dialects in the Custom Languages field, though.
Edit: I found mentions of the Malay Indonesia (ms-ID) locale in other projects as well. For example:
https://man.archlinux.org/man/extra/perl-datetime-locale/DateTime::Locale::ms_ID.3pm.en
Edit 2: Rewording some parts
