Tibetan translations in many examples are interpretations not literally translations
This conclusion has been famously reached based on only a few example such as Nirvana. However, even Burnouf who recognizes these occasions of interpretation concedes that Tibetan translation of Sanskrit texts is one of "perfect literlality; when it is applied with rigor, a Tibetan version represents every feature of the Sanskrit text." Most days I spend a bit of time translating Sanskrit and Tibetan texts and most often the Tibetan translations are very close readings. However, I am ignorant of the history around the Sanskrit influence on Tibetan grammar. Anyways, maybe this should be another thread. In terms of the original post I prefer to do my practice in the language it was composed in, so mostly it is Tibetan however if reciting the Heart Sutra for example I prefer to do it in Sanskrit.
To make it clear, I fully appreciate work of tibetan lotsawas during 2 periods of translation process. It was tremendous effort, that became great gift for us "western barbarians" In my point of view it is extremely difficult to realize kyerim stage not using native language that influenced on condition of someone's channels (I recommend: "The Generation Stage In Buddhist Tantra") by Gyatrul Rinpoche. Moreover semantic field of (e.g.) word "dkar po" never (if your first language is Eanglish) became for you "white" and so on.