Languagefiles are kept separately from the modules. For non-EN languages it's good practis to update the pack now and then, since they are evolving during time. If you were using EN, then my guess is you are using Moodle 1.1: the lesson didn't existed at releasetime of 1.1, so no lessen-words in that release. When installing a new module, you'ld better update the languages you use too.
Languagepacks are always backwards compatible and contain the words of all modules, not only the installed ones.