You can only do AND/OR (Boolean) searches in the word search boxes not when you are using the filters. With word searches you are looking at recipe title, book title, author, ingredients and categories, so you will find recipes containing French bread, German mustard, etc not just those with a French or German tag. I'm not sure how helpful this will be.
Any reason why you wouldn't just do a search using the filter ethnicity French and then do a search using the filter ethnicity German?