The biggest problem with concurrent translation -- in which everything is said in both languages -- is that students are aware they hear what is being said in whichever language they are personally more fluent in, so they simply don't pay any attention to what is said in the other language. For example, if a student's native language is Spanish and the teacher gives a particular instruction in both Spanish and English, the student listens to the Spanish version while tuning out the English version. The student then fails to improve his English-language skills.
Researchers Sharon Ulanoff and Sandra Pucci did a study on the relative effectiveness of concurrent translation and preview-review methods for English as a Second Language programs. Preview-review is a method in which the actual lesson is given in the second language, but the preview and review exercises are given in the students' first language. The test showed that students were able to remember much more second-language vocabulary when the preview-review method was used as opposed to the concurrent translation method.
According to an article by the Lebanese American University, the concurrent translation method can cause problems for learners due to the ambiguities of language. For example, there are five distinct Spanish verbs that can be used to translate the English verb "to pour," but none of them is actually a perfect match. A teacher attempting to translate a sentence about pouring something would need to be fluent enough to know exactly which of the five Spanish verbs would normally be used in that context. A mistake would be easy to make, leading to confusion on the part of the learner.
The decision about which method to use in bilingual classrooms depends on the goal of the course. If the goal is primarily to give the students skills in a particular subject, such as math or history, then it makes sense to emphasize the use of their native language because this is the easiest and most effective way to teach them the material. If the goal is to teach them the second language, then it makes more sense to emphasize the second language. However, even though concurrent translation methods are widespread in bilingual classrooms, they have been shown to be ineffective. Even though they actually tend to emphasize the second language, they still don't produce successful results compared to other methods.