Translated text is detectably different from text originally written in the target language. This phenomenon — translationese — has been attributed to interference from the source language, to simplification strategies, to socio-cultural factors. The explanations are fragmented because no unified account connects them.
Translationese is predicted by translation task difficulty (arXiv:2603.12050). The harder the translation task — measured by information-theoretic metrics based on LLM surprisal — the more the output diverges from native target-language text. The distortion is not a flaw in the translator's competence. It is a rational response to cognitive load.
The difficulty has two components: source-text complexity (how hard the original is to process) and cross-lingual transfer difficulty (how hard the mapping between languages is for this particular segment). Cross-lingual transfer difficulty contributes more than source-text complexity. The distortion in translated text comes primarily from the difficulty of the mapping, not the difficulty of the source.
Information-theoretic indicators — surprisal-based measures computed from language models — match or outperform traditional features (syntactic complexity, lexical diversity) in predicting translationese for written text. For spoken text, the information-theoretic advantage disappears. The mode of production constrains which difficulty metrics are informative: writing allows the translator to manage cognitive load through revision; speaking does not.
The implication reframes translation quality assessment. Translationese is not error. It is the measurable trace of difficulty, as rational as flinching from pain.