Descriptive linguistics has in recent years emphasised the equal value of all language varieties and has discredited prescriptivist attitudes to language use. However, descriptivist approaches are inappropriate where evaluation of language use is inevitable: in pedagogical contexts and where public language use by language professionals is involved. The present paper argues that the basis of evaluation for language use is communicative efficiency, and that the observation of language norms, representing habitual, unmarked language use, plays an important role in communicative efficiency. Relevance theory provides a convenient framework for the evaluation of various kinds of language use, including translation. Translators as public and professional language users are expected to be able to operate a wide range of language norms and use them as required by the communication situation. Most translation situations require that the translation should follow prevailing language norms. While translation quality and usability are different concepts, it is desirable that translators should provide the best quality possible under the given circumstances, since translations have an important influence on the quality of language use in general.