Building on what lordingtar said, you could try to get them involved in whatever media consumption/ cultural activities you can find. For instance, you could invite them over to watch movies in German (I assume that's the target language). You could also being them to fun events where you know the language will be spoken. If you have friends who speak German you could invite them and your Chinese language partners to the same event/activity/party and encourage them to interact. All of these methods involve a social component, which is the strongest motivator for many people (remember, they are in a foreign country: they may be feeling lonely or lost, even with their Chinese fiends).

Building on what lordingtar said, you could try to get them involved in whatever media consumption/ cultural activities you can find. For instance, you could invite them over to watch movies in German (I assume that's the target language). You could also bring them to fun events where you know the language will be spoken. If you have friends who speak German you could invite them and your Chinese language partners to the same event/activity/party and encourage them to interact. All of these methods involve a social component, which is the strongest motivator for many people (remember, they are in a foreign country: they may be feeling lonely or lost, even with their Chinese friends).