Many (probably most, if not effectively all) sentences in Tatoeba are uncopyrightable and labelling them as CC BY 2.0 is confusing

[aucampia]

From Circular 33 of the United States Copyright Office:

Words and short phrases, such as names, titles, and slogans, are uncopyrightable because they contain an insufficient amount of authorship.

This means that many sentences, such as "How are you doing?", would be uncopyrightable. However "How are you doing?" is labeled as licensed under CC BY 2.0, so if I used it I would have to credit Zifre, which is quite absurd to being with as a requirement and likely also not enforceable. It should rightly be labeled CC0 and the contributor should also be told their contribution will be under CC0 mostly as a matter of clarification. I suggest you switch to only allowing contribution under CC0 and then allow replacing of old content that is incorrectly licensed with CC0 content. The sooner you fix this the less work there is to clean it up.