Issue #286: tests broken with German language settings (conservancy/kallithea)

[domruf]

New issue 286: tests broken with German language settings
https://bitbucket.org/conservancy/kallithea/issues/286/tests-broken-with-german-language-settings

domruf:

The recent i18n fixes seem to have broken some tests if LANG=de_DE.UTF-8.

i.e. ```test_create_on_top_level_without_permissions``` contains
```response.mustcontain('<span class="error-message">Invalid value</span>')```

Since I don't know how to translate strings in pytest I create this issue.

BTW using ```with test_context(self.app):``` still results in 
```TypeError: No object (name: context) has been registered for this thread``` when trying to use ```_("Text to translate")```