Fix error when previewing evaluations with undecided main language

[Jakobus400]

#2555 Preview of evaluation form failed with undecided language. Fixed by checking in render_vote_page()

[niklasmohrin]

we also still need a test that checks that accessing the page works :)

[niklasmohrin]

one more thing: the magic "fix #..." line should be in the PR description, not the PR title or commit message. You can still change this, and then you will see that this PR will be automatically linked to the issue in the "Development" section on the right of the page.

[niklasmohrin]

code looks good, but still needs tests

[janno42]

Hi @Jakobus400, do you want to continue with this or should we take over?

[richardebeling]

thanks

[niklasmohrin]

I agree with Richard. Other than that, I have one small suggestion. Thanks!

[niklasmohrin]

Ah, our linter now complains that the function has too many local variables. I think for this PR it is fine to disable this lint in this function. You can put a pylint: disable=too-many-local-variables comment in the function (there are other examples of this in the code base and you can run ./manage.py lint pylint to check whether it worked)