WPML

On sites running WPML, each index has an Index Language setting listing your active languages. Pick one or more to limit the index to them, or leave it empty to index every language. See Create an index.

Text you enter in SearchSync, such as search box placeholders and button labels, can be translated with WPML String Translation.

Other translation plugins aren't supported out of the box. Developers can add languages with the tangible_searchsync_index_languages filter.