Adds the language filtering option on taxonomy to post types connections

[macharest]

Fixes the issue where language filtering did not work on relationships between taxonomies and post types.

For example, the following query would return the posts associated to each category in the default language (english in my case), even if the categories are in french:

categories(where: {wpmlLanguage: "fr"}) {
  nodes {
    id
    name
    posts {
      nodes {
        title
      }
    }
  }
}

I expected to received the linked posts in the same language as my category.

My fix allows us to add the same wpmlLanguage filter on the child queries which will return the posts in the same language as the category:

categories(where: {wpmlLanguage: "fr"}) {
  nodes {
    id
    name
    posts(where: {wpmlLanguage: "fr"}) {
      nodes {
        title
      }
    }
  }
}

This fix works for the relationship between posts and categories, but il also works with any relationships between custom taxonomies and custom post types.

In an ideal world, we would not have to do this, but this is the fix I came up with. Feel free to do what you want with it.

[rburgst]

Hm. I tried your initial query and in my setup I only get the corresponding posts in the translation. Did you translate the categories in the first place? I do have the categories translated and the posts are assigned to the translated categories. That way I only get the posts in the correct translation.

In other words, I have a hard time testing whether your PR works.