When you provide a string in a filter - the endpoint will only return items where the english result matches. This should be documented, and how to filter on another language should be documented ie "How to filter on a specific language".

For example if I filter on the tours with ?body[like]=%25einmalig%25 I will get no results.

To filter on a foreign language I must filter on ?body_de[like]=%25einmalig%25, and then I will get the Tiger beach tour.

https://demo.magnolia-cms.com/.rest/delivery/tours/v1?body_de[like]=%25einmalig%25

(See https://jira.magnolia-cms.com/browse/MGNLREST-181 for extended discussion)

(cc mgeljic)