In Japanese, the sign more obviously means "Yukkuris who can only speak slowly," or "Yukkuris who can only speak Yukkuri [language]," but could maybe be stretched to include "Yukkuris who can only say Yukkuri." I have no idea which one Remiryaaki intended, or if he was doing a joke on the double meaning. I went with "Easy-talking Yukkuris" because it seemed like a translation that could also be interpreted in a number of ways.
**Ee: Of course, they're saying "Yu" in Japanese.
BTW, I've gone through so many different interpretations of just _what_ is happening in this comic. At the moment, my favored interpretation is: Anon bought two Easy-Talking Yukkuris. Then when they had babies, the babies talked normally, so he returned the parents as defective (like, since the Easy-talk gene(?) didn't get passed on to their children, the parents weren't genuine Easy-talkers). Even though the children weren't bought from the store, he didn't want them, so he gave them to the store, and they're in the jar marked "Sample". YMMV. (In this case, like a Hummer and a Prius!)
Damn you, Remiryaaki and your intriguing yet ambiguous cartoons.
Usually I leave all the "yu"s, but one of my earlier interpretations of the comic was that Mister bought the yukkuris thinking that they'd talk slowly, but returned them when it turned out that the only word they could say was "Easy." (And that the kos in panel 3 were yukkuris that he already had at home, and he bought the easy-talkers to act as their parents. But then I noticed the empty stalk on Reimu's head.)