The CSRF error messages are not translated

[azmeuk]

https://github.com/wtforms/flask-wtf/blob/e9f085dfc1ab951613ed2d31feafc7557ed5f914/src/flask_wtf/csrf.py#L99-L115

Even if those messages should not been seen that often, I do have users reporting that they encountered CSRF errors in production projects. For that reason, that would be nice if the messages be localized.

I suppose there is something to do with field.gettext but starting translating strings in flask-wtf would require to set up a whole i18n stack (catalogs, weblate etc.) like this is done in wtforms.