I have observed that more translation mistakes happen in the question in which we ask our university students to translate from their mother tongue into English. Why is that?
I think translation from English into one's mother tongue is easier. The students faced difficulties because English is their second language. Sometimes they do direct translation which Leads to wrong Grammar.
I think that translation to English must be hard for native speakers of many languages. The complex verb tenses, pronouns, "to be" verb, and rigid structure of the grammar is very hard, I think.
Backward direction- that is L2 to L1 transference, is easier . You can refer to Kroll and Stewart's (1994) Revised Hierarchical Model for further details.
I think that some students lose marks in the translation exam, not because of translation mistakes, but because their writing skill in English is rather poor.