But it's not even a proper English phrase. It has to be "who rests/is resting beside the deep sea".
The Hungarian is akik (plural), not aki. So the corresponding English is "who rest/are resting beside the deep sea."
But that doesn't work in English. It is never correct to say "who rest beside the sea"—in this case, "who rests/is resting beside the sea" is both singular and plural. You can never say "who are resting", just as you must say "who is sleeping" not "who are sleeping."
The people who is resting? I don't think so.
What you are saying may be true for questions. But the above is not a question! It is a... it is a.... relative clause, if I am not mistaken.
"We can catch the people who rest on their laurels" (definitely not 'who rests' on their laurels.)
"We should not awaken those who are resting." (definitely not 'who is resting'.)
It totally does work. What's wrong with "The people who rest/who are resting beside the deep sea" ?
The proper translation (into a coherent English sentence) should be "who are resting beside the deep sea"
i think it is 3rd person plural as in, the roman army, who rest beside the deep sea ?