The translation su agrees in number with the noun which it modifies; their is translated by su, sus, depending on what follows: their father/mother su padre/madre; their books/magazines sus libros/revistas. For their used with parts of the body,
Here is a selection of useful words and phrases you will need in real-life situations while you're visiting Spain and other Spanish-speaking countries...
The Basque autonomous police force is called Ertzaintza. Its members, called ertzainas, wear a uniform of red sweaters and berets, and white jackets. Despite the Ertzaintza's wide range of responsibilities, the Guardia Civil and Policía Nacional still operate in the Basque Country.