The Department of Defense has 40,000 positions requiring foreign language skills, yet 70% are filled by people who lack the necessary language proficiency. Too many language learners fail to reach the desired level of proficiency, or lose proficiency if they do not have continuing opportunities to practice. We propose that adaptive training technology can address this problem and help learners quickly develop and maintain language proficiency. To do this, adaptive training should (a) provide learners opportunities to practice using the language in realistic situations, (b) identify gaps in the learners' language competencies in the context of language use, , and (c) recommend personalized exercises to overcome these gaps. Such an approach offers significant advances over conventional methods that emphasize passive language skills (reading, listening) and basic knowledge of vocabulary and grammar without realistic practice in using the language. We have implemented this approach in ALLEARN (Adaptive Language LEARNing), an open architecture for language learning and retention. ALLEARN provides a simulation-based training environment for practicing language use and assessing language competencies, and recommends personalized learning trajectories that focus on overcoming each learner's individual competency gaps. It is designed to be used in a blended learning curriculum, supplementing classroom instruction. To obtain formative feedback and identify best practices for designing adaptive training, focus groups were conducted with Modern Standard Arabic (MSA) students and instructors at the US Army JFK Special Warfare Center and School. Two classes of MSA students were assigned to either the adaptive version of ALLEARN or a non-adaptive version which followed a fixed curriculum sequence. The study investigated a variety of outcomes between these conditions, such as improved performance on oral and written examinations, time-on-task, and training usage behaviors in the context of blended learning. The findings offer implications for best practices of competency-based adaptive training and language learning technology.