fix tests with language option

[cehbrecht]

Overview

This PR fixes the current test failures with language option.

Related Issue / Discussion

See PR #510.

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[cehbrecht]

@davidcaron the CONFIG.set overwrite of the config does not work in the tests anymore when using pywps.tests.client_for. To get the tests working again I have added the language=en-US,fr-CA to the test config.

[cehbrecht]

@davidcaron I'm merging this to get the test suite running again. Maybe we can fix the config issue with another PR.