Clarify language tags.

Re #349

CONTRIBUTING.md

@@ -42,7 +42,7 @@ Translations to new languages are always welcome, especially if you can maintain
 
 - Check existing issues to see if a translation is in progress or stalled. If so, offer to help.
 - If it is not in progress, file an issue for your language so people know you are working on it and we can arrange. Confirm you are native level in the language and are willing to maintain the translation, so it's not orphaned.
-- To get it started, fork the repo, then submit a PR with the single file README-xx.md added, where xx is the lowercase language code. (Use standard two-letter ISO language codes, i.e. the same as is used by Wikipedia, not the code for a single country.)
+- To get it started, fork the repo, then submit a PR with the single file README-xx.md added, where xx is the language code. Use standard [IETF language tags](https://www.w3.org/International/articles/language-tags/), i.e. the same as is used by Wikipedia, *not* the code for a single country. These are usually just the two-letter lowercase code, for example, `fr` for French and `uk` for Ukrainian (not `ua`, which is for the country). For langauges that have variations, use the shortest tag, such as `zh-Hant`.
 - Invite friends to review if possible. If desired, feel free to invite friends to help your original translation by letting them fork your repo, then merging their PRs.
 - Add links to your translation at the top of every README*.md file. (For consistency, the link should be added in alphabetical order by ISO code, and the anchor text should be in the native language.)
 - When done, indicate on the PR that it's ready to be merged into the main repo.