Improve support for RTL languages.

* Update transifex resource name.
* Use RightToLeft layout for Arabic language.
* Use better names and comments on i18n section.
* Use unicode to initialize widgets with text that otherwise (str) will
  fail when Arabic (and most likely any other language with non-ascii
  characters) is used.

Closes #5289, #6033.
Merge remote-tracking branch 'refs/remotes/ivan/feature/6033_support-for-RTL-languages' into develop