About the tool: BCP 47 language tags are built from subtags in the IANA Subtag Registry. This tool helps you find or look up subtags and check for errors in language tags. It also provides information to guide your choices.
Latest changes: I reworked the informational text that accompanies macrolanguages, their encompassed languages, and extlang subtags. As part of that, I changed the code to allow for highlighting of specific cases. For example, where legacy may dictate that the macrolanguage subtag (zh) is more useful for Mandarin Chinese than the more specific tags (cmn or zh-cmn).
I simplified the intro to the page, but added a link to the new article Choosing a Language Tag, which provides useful step-by-step guidelines on creating language tags.
I also changed the user interface somewhat. The input fields are easier to work with and take up less vertical space. Also, you can now submit a query by simply hitting return after typing into a field. I had originally required you to click on a submit button so that all values in other fields would be retained when the answer is shown – this was so that while checking various subtags you could build up a language tag in the Check field for later checking. I just found that the annoyance of continually having to resubmit after forgetting to click on the submit button wasn’t worth the extra functionality (and I was also encouraged to do so by feedback from Bert Bos).