Sorry, that link is not pointing to the page I expected.
In v5 it worked like this:
1) Give your custom field a name in capital letters only (e.g. MYCUSTOMFIELDLABEL), so Acymailing will handle it as a language constant.
2)Edit each of your languages in Configuration->Languages and add custom translations
For example put in the field label: MYCUSTOMFIELDLABEL
And in the custom translations part for each language put this: MYCUSTOMFIELDLABEL="My translated label"
Again, that's how it worked in v5 and I have not tested this in v6.