Plugins need to be enabled through the main network admin area, but then can be either forced active on other sites or optionally activated per site as desired. Searches are language agnostic, they simply match text strings regardless of what languages are configured. If one searched for “recetas” on an English language site that happened to have “recetas” in a post somewhere, search will make the match even if English was the language setting through out.

It’s conceivable for code to take search terms, translate through a third party API, then include the translation as part of the search query. AFAIK, this would need to be custom coded. It’s worth doing a plugin search before trying to custom code something like this.