I have added a bunch of additional new features to my lookup tool to help with choosing language tags. There is additional information available when you look up subtags (such as what to use if the subtag is deprecated, and what subtags macrolanguages enclose, etc.), and more tests on well-formedness with clearer explanations of the problem. Example.
This should make it a lot more useful to people who haven’t read BCP 47 and want to create language tags. Hopefully, in a short while, I’ll also write and link to an article that describes how to use subtags from the ground up in a procedural way, that will complement the tool.
For further assistance, you can now link from a language subtag result to the SIL Ethnologue, to make it easier to check whether that subtag really does refer to the language you were thinking of.
In addition, script subtag results link to Unicode blocks in UniView.