When reviewing metadata if the newly downloaded metadata has no langu…

…age but there is an existing language, ensure it is preserved. Fixes #1959981 [book metadata download fails language attribute](https://bugs.launchpad.net/calibre/+bug/1959981)

src/calibre/gui2/metadata/diff.py

@@ -547,6 +547,13 @@ def apply_changes(self):
             if val != self.initial_vals[field]:
                 widgets.new.to_mi(self.current_mi)
                 changed = True
+        if changed and not self.current_mi.languages:
+            # this is needed because blank language setting
+            # causes current UI language to be set
+            widgets = self.widgets['languages']
+            neww, oldw = widgets[:2]
+            if oldw.current_val:
+                self.current_mi.languages = oldw.current_val
         return changed