dovecot/fts: warn on stopwords filter with multiple languages

mail-server/dovecot.nix

@@ -153,6 +153,20 @@ in
       }
     ];
 
+    warnings =
+      (lib.optional (
+        (builtins.length cfg.fullTextSearch.languages > 1) &&
+        (builtins.elem "stopwords" cfg.fullTextSearch.filters)
+      ) ''
+        Using stopwords in `mailserver.fullTextSearch.filters` with multiple
+        languages in `mailserver.fullTextSearch.languages` configured WILL
+        cause some searches to fail.
+
+        The recommended solution is to NOT use the stopword filter when
+        multiple languages are present in the configuration.
+      '')
+    ;
+
     # for sieve-test. Shelling it in on demand usually doesnt' work, as it reads
     # the global config and tries to open shared libraries configured in there,
     # which are usually not compatible.