Based on the level of respect or formality, you can say "Where did you learn Korean?" in Korean in 3 ways:

• Informal: hanguk-eo-reul eodi-eseo baewotseo? (한국어를 어디에서 배웠어?), hanguk-eo-reul eodi-eseo baewotni? (한국어를 어디에서 배웠니?)
• Formal: hanguk-eo-reul eodi-eseo baewotseupnigga? (한국어를 어디에서 배웠습니까?)
• Very Formal: hanguk-eo-reul eodi-eseo baewosiotseupnigga? (한국어를 어디에서 배웠습니까?)

- where: eodi (어디), eodi-e (어디에), eodi-eseo (어디에서)
- you (subjective): neo-neun (너는), dangsin-eun (당신은), dansin-ggeseo-neun (당신께서는), ne-ga (네가), ni-ga (니가) - omitted here
- to learn: baewuda (배우다)
- Korean: hanguk-eo (한국어)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.