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 (which can take a lot of time, however).
Here are some links to lists of those codes.
- You can view by code, name, scope and type.
- Raw list
- Only ISO 639-1, but with the ISO 639-2/3 added. With links to locally added translations and information about the language.
- List of ISO 639-1 codes
- Same as OmegaWiki list, but with less links.
- List of ISO 639-3 codes
- Links to subpages with a list and a few translations.
- BCP 47 Code Validator
See also 
- Special:SiteMatrix – a list of existing Wikimedia wikis