I succesfully used userforms and mathspamprotection in numerous projects. But now i’m running into an issue with a multilingual site.
The question of mathspamprotection is always in the default language (in my case Dutch). When navigating to the english or french version of the site, the question is still in the default language.

Does anybode have a solution or workaround?

Seems like it should work: https://github.com/silverstripe/silverstripe-mathspamprotection/blob/master/code/MathSpamProtectorField.php#L138

Nothing jumps out at me as to why not, I can only guess that something is resetting the language or that it's not being detected correctly (or relies on the default locale, not the current locale).
You'll have to submit an issue or pull request to fix it I think.

This line in my _config.php triggers the language from the editablespamprotectionfield
18n::set_locale('nl_NL');

It always uses that value. If i uncomment this line, it always use en_US

This line in my _config.php triggers the language from the editablespamprotectionfield
18n::set_locale('nl_NL');

It always uses that value. If i uncomment this line, it always use en_US

PROBLEM SOLVED

I ended up putting this line of code in the init function of my custom pagetype’s controller (contactpage)
i18n::set_locale($this->Locale);

Now it’s showing the mathspamprotectortext in the correct language when switching languages in a multilingual website with translatable and translatabledataobject.