Add more i18n labels

[xfq]

The W3C i18n WG uses a set of labels for tracking issues in W3C groups (WG/CG/IG etc.) and the WHATWG. (See some examples in CSS, HTML, Web Audio API, WebAssembly, WebXR, and MathML.)

The labels include a i18n-tracker label, a i18n-needs-resolution label (documented here), and a few i18n-*lreq labels. The effect of the i18n-*lreq labels is to notify experts in the language enablement groups (such as clreq and jlreq) on activities in the issues with these labels.

The HTML repo currently doesn't have the following labels:

• i18n-eurlreq
• i18n-mlreq
• i18n-sealreq
• i18n-tlreq
• i18n-afrlreq
• i18n-elreq
• i18n-amlreq

Maybe not all of them are useful for current issues in HTML, but according to https://github.com/whatwg/html/issues/5121 , I think at least these two labels are needed for that issue:

• i18n-mlreq (for Mongolian)
• i18n-amlreq (for American languages)

I can submit a pull request against LABELS.md and labels.json, if it does not require my employer to sign the participation agreement.

[annevk]

It does not require that and it'd be appreciated. I updated labels.py in #184 to make rolling this out a bit easier.