fix(developer): Select BCP47 Code dialog inconsistencies

Fixes #8157.

There are some limitations in this dialog as it stands; we will rework
it completely when we revisit BCP 47 tagging.

However, for now, this addresses some strange behaviours around the
Reset button and the displayed output:

* The Reset button will only be enabled if you change the text in the
  Language Name field.
* The messaging around a missing language subtag is clearer -- tells
  the user that they need to enter something there.
* The Reset button will now never insert default text strings into the
  Language Name field.

One example of something that is slightly weird, is if you enter a
two-letter region subtag into the script box, it will show what looks
like a valid tag in the output box (e.g. `en-Au`), but it will be
considered invalid, because that two letter subtag needs to be entered
into the region box.