Fix preview and add pl_PL locale

This PR adds Polish locale and fixes TeX rendering in preview pane, which was mentioned at https://meta.discourse.org/t/mathjax-plugin-supports-math-notation-using-latex/12826/19?u=lidel
Reference:
https://github.com/discourse/discourse/blob/f66c39c63bfa57318f742b672b02a02ab18b68a3/app/assets/javascripts/discourse/lib/plugin-api.js.es6