Multi-language SEO is the practice of creating and optimizing content in multiple languages so each version genuinely ranks well in its target market — which requires more than direct translation. It means researching how people in that specific language and market actually search, adapting content to local idioms and cultural context, and optimizing for keywords that reflect real local search behavior rather than a translated version of your primary market's terms.
This is distinct from (but connected to) international SEO, which covers the broader technical and market strategy — multi-language SEO is specifically about getting the content itself right for each language.