I have a list of entities which all support one or more languages.
I have a parent entity which also has a list of required languages.
I would like to know which of the languages if any required by the parent item that all of the children does not support.
Here is one way to do it. You first create a CROSS JOIN of the content IDs and the required languages (to get all combinations that are required) and then you simply look for missing values and output the distinct language IDs of them.
SELECT DISTINCT ParentRequiredLang_IDs AS MissingLanguages FROM (SELECT i.ChildContent_ID, j.ParentRequiredLang_IDs FROM Table_1 i CROSS JOIN Table_2 j) a LEFT OUTER JOIN dbo.Table_1 b ON a.ChildContent_ID = b.ChildContent_ID AND a.ParentRequiredLang_IDs = b.Lang_ID WHERE b.Lang_ID IS NULL