Translation:The people do not stop expressing how they feel.
Interesting. Apparently this has changed not too long ago:
The ox knows its owner, and the ass its master's crib; but Israel does not know, my people does not understand.
(Old Testament, Revised Standard Version, 1952 —Isaiah 1:3)
Here 'people' is like 'povo' in Portuguese, not like 'pessoas'.
I don't think that the OT is a really good source for standard English. However, this is what I have found:
"The ox knows its owner and the donkey its master's feeding rack; but Israel does not understand; My people show themselves lacking in discernment". The Revised Berkeley version in Modern English - 1986 edition.
OK, so it seems something changed between 1952 and 1986. I looked up a lot of translations and all the modern ones have the verb in plural. Whereas the Revised Standard version has the verb in singular, the New Revised Standard Version (1989, 1995) has plural. Of course the real old ones have 'doth' or 'hath', which I think is also singular.