Function resolve_language 

pub fn resolve_language(cli_lang: Option<&str>) -> Language

Resolves the language from CLI flag, env var, or system locale.

Resolution order:

1. cli_lang — value of --lang flag (if provided)
2. CONTEXT7_LANG environment variable
3. sys_locale::get_locale() — BCP 47 locale (e.g. "pt-BR")
4. Default: English