Search results inconsistent

The search results when using the default wordpress search, and without any plugins are very inconsistent and most of the time the translated pages are not being found by the search.

For example we have a post with the word ‘collection’ in the title for the default english version, and then we have it translated to ‘collectie’ for the dutch version of the site. When we search using the dutch website we can’t find the post by searching for ‘collectie’, but we can find it by searching for ‘collection’.

It seems like the search is still just searching for the original language for most post the posts. But some other posts can be found using the translated titles.