Add support for more languages

**Issue by [jachym](https://github.com/jachym)**
_Sunday Jan 03, 2016 at 20:57 GMT_
_Originally opened as https://github.com/PyWPS/pywps-4/issues/79_

---

Currently, in PyWPS language "en-US" is hardcoded. For adding support for more languages we need:
- [ ] To define configuration of supported languages in configuration file
- [ ] Use possibly existing i18n python-based tool around and i18n gettext seems to be it https://docs.python.org/3/library/gettext.html#internationalizing-your-programs-and-modules

Should be relatively easy, guessed time: 40 hours