Wiktionary
n. The quality or property of being untranslatable; inability to be translated.
Wikipedia
Untranslatability is a property of a text, or of any utterance, in one language, for which no equivalent text or utterance can be found in another language when translated.
Terms are, however, neither exclusively translatable nor exclusively untranslatable; rather, the degree of difficulty of translation depends on their nature, as well as on the translator's knowledge of the languages in question.
Quite often, a text or utterance that is considered to be "untranslatable" is actually a lacuna, or lexical gap. That is, there is no one-to-one equivalence between the word, expression or turn of phrase in the source language and another word, expression or turn of phrase in the target language. A translator can, however, resort to a number of translation procedures to compensate for this. Therefore, untranslatability or difficulty of translation does not always carry deep linguistic relativity implications; denotation can virtually always be translated, given enough circumlocution, although connotation may be ineffable or inefficient to convey.