Wikimedia does not decide for itself what is a language and what is a dialect. We follow the ISO 639
standard. Every Wikimedia language edition is required to have a valid ISO 639-1 or ISO 639-3 code. Every language with an ISO 639-1 code also
has a ISO 639-3 code, but not vice versa.
If a 639-1 code is available, that one must be taken, otherwise the ISO 639-3 code. If none available, the language has no future on Wikimedia, unless you request a code
(However, this can take a lot of time).
Here are some links to lists of those codes.