The Accessible.js config loads accessibility-menu extension which in
turn loads mathjax-sre.js library. There is a bug in this library
https://github.com/zorkow/speech-rule-engine/issues/182 that makes it
fail in RequireJS environments. This has been reported upstream as
https://github.com/mathjax/MathJax/issues/1854
As an immediate solution for now, this patch changes the default MathJax
configuration so that it does not load accessibility extensions at all.
While working on the issue, I noticed that the current implementation of
the method filter_mathjaxloader::map_language_code() is suboptimal and
does not work well for many Moodle language packs such as pt_br or de_du
and other variants. It did not give particularly wrong results. It just
fell back to the default English too early without trying more hard.
This patch improves the mapping logic and adds a unit test for the
method.
David Mudrák