This chapter delves into the critical role of lexis in L1 and L2 acquisition, exploring how vocabulary reflects language development and impacts text quality. The mental lexicon forms an intricate web of semantic connections, with bilinguals processing words differently based on proficiency. Low-proficiency learners rely on L1 translation, while advanced speakers strengthen direct links to L2 vocabulary. Research shows that both languages remain active during lexical tasks, shaping bilingual cognition.

Lexical richness is analysed through key models, including Crossley’s and Jarvis’. Advances in natural language processing have enabled automated evaluation tools like Coh-Metrix and TAALES, enhancing lexical analysis.

As bilinguals progress, their writing becomes more diverse and sophisticated, though L1 and L2 development may diverge in features like word concreteness. Formulaic language is also crucial – high-proficiency L2 writers use more native-like phrasal structures. By examining lexical acquisition, this chapter highlights its significance in bilingual proficiency, providing insights into how vocabulary shapes linguistic competence.