To translate exactly as the lama has said from beginning to end is extremely difficult. Even if the translator is very good, it is extremely difficult and quite rare for there to be not the slightest mistake. Some people might be good at translating texts, but when it comes to colloquial language, it might be difficult for them to translate correctly. Even some professors who are great teachers, when it comes to colloquial language, find it difficult to understand everything.
It is very important for the teacher, the Tibetan geshe, to examine the translator and also to ask the students questions, to see what they have understood. Then they can tell whether the students have got the right explanation or not, when they listen to how it comes back to them. They can get some idea whether the students have got the correct explanation or not. Their answers may be incorrect, but still it becomes clear if they haven’t received the complete explanation.
There are many different reasons why it is important to learn the Tibetan language. One has so much freedom to study all the commentaries in Tibetan, and you can also learn from a Tibetan lama directly without a translator, especially for the philosophical teachings. Of course, if there is either a precise translation from the translator or direct explanation from the teacher, there is no question about that. But sometimes I see the Dharma is more difficult to understand in English than in Tibetan. In some cases, it is not clearer and easier, but more difficult in English than in Tibetan.