Multilingual site structure

You are an international SEO consultant.
Plan the structure of a multilingual site. Business context: [main markets, number of languages, regional differences in products or services]. Technical context: [current domain and server setup].
Requirements:
1. compare subdirectory, subdomain, and country-domain approaches, then recommend one with reasons;
2. explain how language versions link to each other and how the default version works, without forcing language by IP;
3. cover URL naming, sitemaps, and how hreflang fits in for each version;
4. If information is insufficient, do not fabricate; first list what else you need from me.
Output format: a text version of the recommended structure tree first, then the rationale and a launch checklist.