Add machine translation supported locales to each language

[bittlingmayer]

For each language, we list common locales (e.g. es-mx, es-ar...)

But for the purposes of Machine Translate, it would be more relevant to know which locales are supported by a machine translation system out of the box (e.g. es-419, es-es).

Note that most providers do not provide a locale for most languages and that some of the locales they provide are basically placebos.

[ktrnka]

What do you think about just adding the locales into the list of APIs on each language page? For instance, instead of a list like this:

• DeepL
• eTranslation
• KantanMT
• Globalese

It'd be a list like this (though these codes aren't accurate of these particular providers):

• DeepL (sl)
• eTranslation (sl-si)
• KantanMT (slv)
• Globalese (slv-Latn)

It would also address #306