Should international SEO use country-based or language-based subfolders?

Short answer: The decision follows the goal. If you serve several countries that share a language under different commercial conditions (pricing, regulation, delivery), you need country-based subfolders (/de-de/, /de-at/); if one operation publishes to language communities, language-based folders (/de/, /en/) are enough.

Decision criteria

• Commercial differentiation: if prices, product range, or legal copy change per country, the country structure is mandatory.
• Content capacity: country versions demand ongoing maintenance per market; with limited capacity, start language-based and split later as a market grows.
• Query divergence: when terms differ strongly between countries sharing a language (Spanish in Spain versus Mexico), a country version creates value.
• Duplication risk: near-identical country versions require flawless hreflang; if you cannot maintain it, the language structure is healthier.

A hybrid model is also legitimate: core markets country-based, the long tail language-based. Migrations run on a 301 map plus updated hreflang, and impact is tracked per market against a baseline.