Stefan O.

Certified German teacher (Goethe-Institut Green Diploma), licensed Goethe examiner for A1-C1, eight years in the classroom including years of C2 courses for adults, and two years as a Duolingo course contributor (1,000+ hours on this platform, every contract rated 5.0). I also build software. Four things I do: GERMAN LOCALIZATION AND LINGUISTIC QA. Software strings in any format (JSON, YAML, PO, XLIFF, ARB, .strings, spreadsheets), UI copy, learning content, marketing text. Placeholders and plurals intact, length checked in context, register and terminology consistent, with a short notes file on anything ambiguous in the source. Also EN-DE editing, transcreation, and review of machine or AI output. Subject areas I know from the inside rather than from research: travel and scuba diving, biology and natural history, language learning, and software. LANGUAGE DATA FOR AI. German and English LLM evaluation, prompt and response writing, rubric grading, error annotation, test-set creation, CEFR-level labeling (I grade real exams at every level), and curriculum design for language products. Two years building Duolingo's German-for-Chinese-speakers course: sentence writing, grammar sequencing, story testing, feedback loops with the content team. GERMAN LEARNING CONTENT AND EXAM PREPARATION. Course and lesson design at any CEFR level, exam-style tasks, and examiner-grade written feedback on Goethe-Zertifikat writing tasks (B1-C2). Async: you send the text, you get a marked-up version with the official criteria applied and a short plan of what to fix first. WEB AND MOBILE DEVELOPMENT. JavaScript/TypeScript, Python, PWAs, Android, Firebase, Chrome extensions, and AI automation with Claude. My own project: DuoNat (duo-nat.web.app), a biodiversity learning game built on iNaturalist data. How I work: async by default. You write the scope, I confirm it in one message, I deliver early with a short "what to check" list. I use AI tools in my process and say so; everything is reviewed by me before it reaches you. MA in Biology (University of Cologne). RAID-certified divemaster with 300+ dives, and four years traveling and diving across Asia and Latin America. Based in Hong Kong, working across time zones.