Symptom
Put Korean in an environment variable *key* and somewhere along the chain (shell, PaaS console, os.environ) it fails to take or comes out mangled. The key, not the value.
Cause
How env var keys are encoded is guaranteed differently by every platform and shell. Values mostly get through; keys have paths where they don't.
Why this bites non-ASCII users
It's tempting to name config in your own language when the values are already in it. Values ride through as opaque bytes most of the time. Keys are parsed, matched, and normalized by every layer, and at least one layer will assume ASCII.
Fix
Keys are Latin letters, uppercase, underscores. If you need to group by prefix, the prefix is Latin too.