An increasing number of schools offer bilingual programs, where lessons are taught in more than one language. Several theories state that bilinguals have greater metalinguistic awareness than monolinguals. We investigated whether this greater metalinguistic awareness is also related to an increased ability to understand an unknown language. To measure metalinguistic awareness and the ability to understand text written in an unknown language, we designed the Indonesian Language Test (ILT). The ILT consists of items regarding a story in Indonesian. Dutch high school students from monolingual and bilingual classes were administered the ILT, a Dutch Language Test, an English Language Test, and a general intelligence test. The ILT showed promising psychometric properties. Bilingual students scored significantly higher on the ILT than monolingual students. Multi-group confirmatory factor analyses showed (i) that ILT measures the ability to understand an unknown language, and (ii) that bilingual students score significantly higher than monolingual students on this ability. Both observations support the notion that bilingual education increases metalinguistic awareness and therefore the ability to understand an unknown language.