On SO en español we found a message that is not localized and also not on Transifex:
Is there a way to localize that string?
EDIT (is not a duplicate)
The duplicate question not applied to this, since that string just wasn't localized (is now localized on SO en español) (we are in the process of localizing it).
The key for that (on Transifex) is: 90a0f41828b69ffe09588c741f067e7f
.