Refactor Locale logic and use Optional for null checks

[SungbinYang]

• Extracted repeated Locale resolution logic into a private method to reduce duplication.
• Replaced traditional null checks with Optional to improve readability and make null handling more explicit.

[sdeleuze]

If think we are prefer to keep the current null check logic as it is rather than paying the cost of wrapping/unwrapping with Optional, the logic in the extracted method is very simple and we tend to prefer to handle this kind of polish within the team, so I decline this PR.