This touches every aspect of language and language is an incredibly complicated concept. If languages are not structured the same way in every aspect meaning gets lost. Of course if they were structured the same way in every aspect it would be the same language. (I am ignoring cryptography here.)
Just consider this example about vocabulary.
Every language is full of semantic lexical gaps. It the semantic gaps between the two languages doesn't match you cannot translate literally. Those gaps tend to be fewer with basic concepts (father, mother, hunger, etc.) and get a lot more when it gets more abstract.
Already one degree of kinship further there are no different words for a female cousin and a male cousin in English. If you translate from a language that makes that distinction within one word you lose some of the meaning it originally had.
Sometimes there are also abstract concepts that just not exist in another language, like the German Gemütlichkeit that has been considered so long as untranslatable because nothing really fit its German meaning that English just adopted it.
You could go to every aspect that makes up a language and see that there are reasons that make a completely lossless translation impossible. Just look at this list of grammatical genders in different languages and its just one aspect that has influence on perception and meaning of the language. That's just one part of grammar, besides grammar there dozens of aspects that have impact on the meaning of what was said or written - or signed.