This is the same problem that we had in MDL-52486. When the language
pack with multiple underscores (such as "en_us_k12") is selected, the
html root element's "lang" attribute is set to
<html lang="en-us-12" ...>
which we then map back to the Moodle lang code using the JS function
replace().
What was missed in MDL-52486 was that when replacing a value (and not a
regular expression), only the first instance of the value is replaced.
So the value "en_us-k12" was sent as the lang parameter for the
core_string, which is invalid PARAM_LANG value and the exception was
thrown.
In case of the user_date.js, there was no actual problem experienced and
probably there is none as the language code is used as a cache key
rather than a PARAM_LANG parameter. But we are changing it too for
consistency.
