mediawiki.language: fix convertNumber( ..., true )

mediawiki.language: fix convertNumber( ..., true )

It was not handling separator unformatting.

In mediawiki.jqueryMsg.test.js fix broken test case:
* Hindi does not use comma as decimal separator!
* The number was not formatted as it would be by the
  formatting code. By definition back-conversion is
  only defined for input that we generate. For other
  input the output is undefined. There are other tests
  with same issue, but I did not touch them now.

Bug: T151198
Change-Id: Icc9bc09675151645055c7e7f706472a15cc81b0f