fix: force language in tests

[andreabergia]

Fixes #1803

[rPraml]

I would like to point out that it might be a bad idea to only run the tests in English.

In principle, I have nothing against changing it like that, but if we change it, then developers might not find errors in local build, that would, for example, appear in production in their language setting.

For example, my machine runs on de_DE and time zone Europe/Berlin and all tests are passing.

/edit: and the reason, why test runs in german locale is, that for the particular error message, there are translations only for "fr" and "zh_CN" ... and a german translation file is missing