Robert Frost famously said that poetry is “what gets lost in translation”. By this he meant that the nuances and aesthetic qualities of language are essential to poetry, and that those features are precisely what do not translate.
The difficulties of translation depend to a great degree on what you are translating and the nature of the two languages involved. A technical manual, which uses simple and precise language, will be much easier to translate than works of imaginative literature, which are steeped in literary traditions, allusions, and figurative language.
A major linguistic obstacle to translation is the problem of conceptual mapping. In simple cases, the relationship between signifier (a word) and concept (signified) is similar across numerous languages. The French “chat”, Latin “feles”, and English “cat” all are based on similar concepts of what constitutes the biological species. For other terms, the conceptual boundaries vary across languages. For example, the boundaries between colors vary from language to language. The Berber or Amazigh term “azegzaw” covers a range of shades that English separates into “blue” and “green”.
A cultural obstacle has to do with terms referring to practices common in one culture but not in another, or where the significance of a term may not be immediately obvious in the target language. For example, the Latin verb “lucubrare” means to work by lamp or candlelight. In twenty-first century culture, we are accustomed to working by artificial light after dark, but this was something unusual in antiquity. Other ancient terms such as “miasma” in the sense of ritual pollution have no real modern English equivalent.