Narrow band voice coders that use vector quantization techniques may suffer from language dependency: the performance of the coder (in terms of speech intelligibility) may depend on the language spoken. For multinational applications, this is undesirable. A test method is proposed that may be used to determine to which extent a vocoder is language dependent. The proposed method, based on a subjective speech intelligibility test in multiple languages, is shown to be feasible by application on known language dependent 'systems': non-native (human) speakers and listeners. The method is shown to be able to significantly prove differences in language dependency, even when using only three languages and nine speaker/listener combinations.