We discuss the results of an experiment that investigates English-French learners' interpretation of quantifiers with detachable restrictions. Such quantifiers are ungrammatical in English. We investigate aspects of interpretation that rely on a highly idiosyncratic interface between grammar and general principles of conversational cooperation in native French. We show that a learning-theoretic challenge of the most severe kind arises in English-French acquisition unless second language acquisition is constrained by very specific relations between syntactic, semantic and pragmatic modules. We therefore argue that the emergence of knowledge of interpretation in English-French interlanguage suggests mandatory, informationally encapsulated computations. This supports Schwartz's (1986; 1987; 1999) contention that inter-language knowledge is constrained by a mental organization in which Universal Grammar provides the contents of a largely universal processor devoted to language.