Is 'Fique de olho' an expression used in Brazil or is it just literal translation of 'Keep an eye'?
Malas are traveling bags, those big ones you fill with clothes and personal belongings and go to the airport.
Suitcases are "maletas" (mala's diminutive).
Luggage would be a good translation, but I believe it doesn't take the plural...
A ‘mala’ is a case made out of plastic, leather, canvas, vinyl or some other sturdy material, with a hinged lid and a handle to make it suitable for carrying clothes and other luggage. So a suitcase or trunk.
It can also be a woman's handbag, more or less synonymous with ‘bolsa’, or the boot of a car, or slang for someone's stomach or abdomen.
In Brazil it can informally also mean a bore (in the sense of a boring person) or it can be a vulgar word for anus or penis.
A ‘maleta’ is a small suitcase, typically one of those on little wheels with an elongated handle that you drag along behind you.
Here's an example: http://milvuelosbaratos.com/wp-content/maleta.jpg
But it can also be used for a variety of other kinds of small suitcases.
On an earlier question, there was a discussion of "leave" vs. "keep" as meanings of "deixar." Could one also say "Deixe de olho nas malas?"