The technology described herein enables users to enrich their vocabulary by annotating and/or automatically translating specific words, which are predicted to be unknown to the specific user. The user experiences the original content enriched with adaptive, smart in-line annotations of unknown words. The technology is tailored to individual users by understanding an individual user's vocabulary in a particular language. As a user consumes content or performs document authoring/editing activities, the system captures language usage patterns, maintained in a private Vocabulary Analytics Store (VAS) for the particular user. Information in the VAS is used as input to a machine classifier that determines whether a word is likely to be known or unknown to a user.