According to the "direct method", which was developed around the end of the 19th/ beginning of the 19th century, only the target language should be used in a foreign language classroom (making it monolingual in L2, so to speak). This lead to a sometimes dogmatic practice to ban students L1 in the foreign language classroom. There are several research findings which show that this approach is not always the best. One can save a lot of time, for example, by translating words rather than explaining them in the target language. Also, translation can be a vehicle for getting a feel for the cultural nuances that go along with a language. And thirdly, often structural (grammatical) features can often be explained best by comparing two or even several languages. So I think a modern language classrooms should not altogether eliminate the students L1, nor any other languages the students may know, like their home language, even if it is different from the other students' L 1, or other foreign languages. On the other hand, especially for advanced students, the "immersion" experience mentioned by colleague Khasianov is a very effective way to make students communicate in L2 even in challenging situations. Probably the best compromise is to have phases when other languages are not allowed and communication in the target language is the main and only objective, an other phases when the target language is analysed and reflected upon and other languages have their place in facilitating this analysis.