If you're already doing so, i highly encourage you to change it globally (even if you have to specify on other place that date info). You will need to change Mesiawiki:copyrightwarning and the copyrightpage. The text below saying it's under GFDL won't be incorrect at all (all of it is under GFDL), but for changing you will need to ask on bugzilla.
About translations from other wikipedias, you're supposed to be able to read from various sources to know and then make your own text so shouldn't be so much a problem unless you directly quote (then you could have a GFDL-only text with dual licensed translation and modifications, quite a © mess...). Maybe a new template: be aware original article source is GFDL-only? Better being creative!
Platonides