Babies can acquire two languages with not much problem. For them there are two ways for calling the same; for example “This is red” or “Esto es rojo”.
It’s an effort for parents anyway, and for teachers.
I would advise to have two specific and different periods of time, one in their native language, and one for the target language.
Some other times the problem is at school. In Spain there’re a lot of bilingual schools, and students have some school subject(s) in the target language, say English for example.
The thing and the problem is that they can learn history in English and have no idea how to talk about history in Spanish, their mother tongue. Any idea? / Photo from: london-2012-the-official-video-game-of-the-olympic-games-screenshots www g4v com. The picture is just for illustration sake