Truthfully, you shouldn't totally ignore ANY girls. We're still a bit away from it, but eventually we will have maps that can only be run by girls who come from a single nation. This means that you'll be running even 2* girls in your teams. These are not rumored to be very difficult maps, but you will still want to have as many girls as you can to fill out your squads.

Certainly, it's more important for now to concentrate on your primary multinational team composition, and 2*s are so easy to replace that you may not even need to keep all of them on hand at this point. But it's worth it to build up the equipment slots/skill levels of your 3*s and 4*s ahead of time even if (like me) you don't want to spend the resources to get them all evolved and leveled. Personally, I'm even working on skilling/slotting a second copy of one 4* who keeps showing up in my gacha draws after the first copy was fully skilled/slotted. It only costs me 1/5 of a flower gem to keep her, and I'll be glad to have her in two of my squads when the time comes.