x-default is a special hreflang value that tells Google which page to show as the fallback when no other language or region variant matches the user's browser settings or location. It is typically set to your homepage or the most universally applicable version of the page. Without x-default, Google will choose a fallback on its own, which may not be what you want.

Generally no. If your entire site is in one language but targets different countries with largely identical content, hreflang is usually unnecessary and adds implementation complexity without significant SEO benefit. However, if you have country-specific pages with meaningfully different content (prices, products, regulations), hreflang on those pages helps Google serve the right version to users in each country.

Yes. Yandex (the major Russian search engine) also supports hreflang tags and honors them for language and region targeting within its index. Bing does not support hreflang — it has its own "content-language" meta tag system. If you need to target Bing users in multiple countries, you will need a separate strategy (e.g., separate domains or subdirectories with content-language meta tags).

Both methods tell Google the same information about language/region relationships. The HTML approach is simpler for small sites but requires updating every affected page. The sitemap approach centralizes all hreflang data in one file, making it much easier to manage at scale. For sites with hundreds or thousands of translated pages, the XML sitemap method is strongly preferred.

Yes. Hreflang works across subdomains (en.example.com, fr.example.com) and completely separate domains (example.com, example.fr). When using separate domains, cross-domain hreflang implementation is valid but requires each domain to reciprocally reference the others in the hreflang set. The same reciprocity requirement applies regardless of URL structure.

If the implementation is invalid (non-reciprocal, wrong locale codes, or broken URLs), Google will ignore the hreflang set entirely and fall back to its own language detection logic. This will not cause a penalty — it simply means international SEO is unguided. You can verify hreflang implementation using Google Search Console's International Targeting report or a dedicated hreflang testing tool.

Use language-only codes (e.g., hreflang="es") when your content is the same for all Spanish speakers globally. Use language-region codes (e.g., hreflang="es-MX") when you have distinct content for specific countries — different prices, local regulations, regional vocabulary, or country-specific products. If you have both a global Spanish page and a Mexico-specific page, use both codes: one pointing to the global page and one to the Mexico page.