"They are here for ages."
Translation:Maen nhw yma ers achau.
This sentence really means something more like "They have been here for ages," right? So how would you say "They'll be here for ages?" I don't think ers would work, but I don't know what else to use
You are right - ers here is looking back in time (and the English translation has now been changed to reflect that.)
Looking forward in time you could use am:
- Byddan nhw yma am oesoedd