Add permission method for wallet_getLocale

[meppsilon]

I added a simple permission for wallet_getLanguage. I wasn't sure how detailed and specific we should make this permission but I see a possible improvement: converting LanguageResult to a enum-like data type where we specify exactly what languages would be returned instead of a returning it as a general "string" type.

[shanejonas]

Looks good, you also need to add the method as a $ref in this file: https://github.com/MetaMask/api-specs/blob/main/metamask-openrpc.json#L7