Based on the level of respect or formality, you can say "I'd like to be fluent in Korean someday" in Korean in 3 ways:

• Informal: na-neun eonjen-ga-neun hanguk-eo-reul yuchang-hage hago sipeo (나는 언젠가는 한국어를 유창하게 하고 싶어)
• Formal: jeo-neun eonjen-ga-neun hanguk-eo-reul yuchang-hage hago sipeoyo (저는 언젠가는 한국어를 유창하게 하고 싶어요), jeo-neun eonjen-ga-neun hanguk-eo-reul yuchang-hage hago sipseupnida (저는 언젠가는 한국어를 유창하게 하고 싶습니다)
• Very Formal: jeo-neun eonjen-ga-neun hanguk-eo-reul yuchang-hage hago sipseupnida (저는 언젠가는 한국어를 유창하게 하고 싶습니다)

- I: na-neun (나는), jeo-neun (저는)
- would like to be: igo sipda (이고 싶다)
- fluent: yuchang-hada (유창한)
- Korean: hanguk-eo (한국어)
- someday: eonjen-ga-neun (언젠가는)

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