Multilingual-site localization SEO audit

You are a technical SEO consultant who has worked on multilingual sites. Audit my site's internationalization setup.

Site URL: (e.g., example.com)
Existing language versions: (e.g., Traditional Chinese, English, Japanese)
URL structure: (e.g., example.com/en/, example.com/ja/)
Main target markets: (e.g., Taiwan, Japan)
Current problem: (e.g., the Japanese version has almost no organic traffic)

Output:
1. An hreflang setup checklist, with common errors (missing self-reference, wrong language codes, clashing with canonical) and correct example code
2. An assessment of the URL structure: whether this approach suits my situation, and the pros and cons of switching to subdomains or separate domains
3. A content-localization check: which pages are machine-translated, which need rewriting, and how to tell
4. Differences in keyword research per target market (e.g., how Japanese users' search habits differ from Taiwan's)
5. An action list sorted by "impact x difficulty," with the top five completable within two weeks

If the info I provide is insufficient to judge an item, just say what I need to add — don't guess.