Issues in MathJax translation

Created by: fred-wang

As indicated in #503 (closed), I've found various issues in MathJax translation that should be fixed.

General bugs:

"MathError" is not translated.
In FontWarnings extension, markdown links are not parsed.
It seems that the selection between "fonts", "STIXPage" or "TeXPage" is done according to whether the availableFont list is ["STIX","TeX"],["STIX"] or ["TeX"]. I didn't realize that "webFonts" was used in combination with "imageFonts" and "noFonts". Thus I tried to set availableFont to ["web"] and it does display the "webFonts" message! I don't think it's too serious, but I've updated my branch to remove that test.
The "webFonts" message is not translated.
"InvalidMathMLAttr". The string ID is displayed instead of the translation. Probably some _() is forgotten in the source code.
"Original" is not translated.
"DoubleBackSlash". This happens with e.g. "\let INVALID". The message is "\ must be followed by a control sequence", but I'm not sure it is quite correct? Should it be "%MathJax v2.3 must be followed by a control sequence", where the argument is e.g. "\let" or other macro definition? If so the string id is not really good either.

German:

French: