As far as I understand, since you recruit celtic troops from the celtic settlements, your roman recruitment building gives you celtic cultural conversion. This changes to italic conversion once you turn a celtic settlement into a minor city and build Roman recruitment 3 or 4(I don't remember which one :D). However getting to this level of recruitment makes you unable to recruit many celtic troops.

I think this system is great. You have to balance between having access to different troops and romanizing. This forces you to create recruitment hubs and cultural hubs.