get_page method doesn't find the post with

When "Different languages in directories" is selected for "Language URL format" in WPML, get_page doesn't find the post with slug set to the pagename.

e.g. http://example.com/en/api/get_page/?slug=abc

returns:

```
{
  status: "error",
  error: "Not found."
}
```

while it works fine with "id" specified like:
http://example.com/en/api/get_page/?id=1
