This update to the Language Subtag Lookup tool brings back the Check function that had been out of action since last January. The code had to be pretty much completely rewritten to migrate it from the original PHP. In the process, I added support for extension and private use tags, and added several more checks. I also made various changes to the way the results are displayed.
Give it a try with this rather complicated, but valid language tag: zh-cmn-latn-CN-pinyin-fonipa-u-co-phonebk-x-mytag-yourtag
Or try this rather badly conceived language tag, to see some error messages: mena-fr-latn-fonipa-biske-x-mylongtag-x-shorter
The IANA database information is up-to-date. The tool currently supports the IANA Subtag registry of 2014-12-17. It reports subtags for 8,081 languages, 228 extlangs, 174 scripts, 301 regions, 68 variants, and 26 grandfathered subtags.