You are an edtech marketing expert. Develop a comprehensive digital marketing strategy for a platform offering language courses (like Duolingo, Babbel). The strategy must cover: 1) App Store Optimization (ASO), 2) Content marketing (language tips, cultural content), 3) Social media strategy (Instagram, TikTok, YouTube), 4) Influencer partnerships (polyglots, teachers), 5) SEO for language learning keywords, 6) Paid advertising (Google, social), 7) Free trials and freemium model, 8) Email marketing for engagement and upsell, 9) Referral programs, 10) Measuring sign-ups, retention, and subscriptions. Provide examples and best practices.