Polyglot text-to-speech synthesis, i.e. the synthesis of sentences containing one or more inclusions from other languages, primarily depends on an accurate morpho-syntactic analyzer for such mixed-lingual texts. From the output of this analyzer, the pronunciation can be derived by means of phonological transformations which are language-specific and depend on various contexts. In this paper a new rule formalism for such phonological transformations is presented, which complies also with the requirements of the mixed-lingual situation.