diff --git a/libgimp/gimpplugin.c b/libgimp/gimpplugin.c
index 0034b74430..4cc901b468 100644
--- a/libgimp/gimpplugin.c
+++ b/libgimp/gimpplugin.c
@@ -898,15 +898,17 @@ _gimp_plug_in_set_i18n (GimpPlugIn   *plug_in,
     }
   else
     {
+      gchar *utf8_catalog_dir = NULL;
+
       use_gettext = GIMP_PLUG_IN_GET_CLASS (plug_in)->set_i18n (plug_in,
                                                                 procedure_name,
                                                                 gettext_domain,
-                                                                catalog_dir);
+                                                                &utf8_catalog_dir);
       if (use_gettext)
         {
           gboolean reserved = FALSE;
 
-          if (! (*gettext_domain))
+          if (*gettext_domain == NULL)
             {
               *gettext_domain = g_strdup (plug_in->priv->translation_domain_name);
             }
@@ -919,17 +921,34 @@ _gimp_plug_in_set_i18n (GimpPlugIn   *plug_in,
                * set by the lib itself; devs must set NULL). See docs of
                * set_i18n() method.
                */
-              if (*catalog_dir)
-                {
-                  g_printerr ("[%s] Do not set a catalog directory with set_i18n() with reserved domain: %s\n",
-                              procedure_name, *gettext_domain);
-                  g_clear_pointer (catalog_dir, g_free);
-                }
+              if (utf8_catalog_dir != NULL)
+                g_printerr ("[%s] Do not set a catalog directory with set_i18n() with reserved domain: %s\n",
+                            procedure_name, *gettext_domain);
 
               *catalog_dir = g_strdup (gimp_locale_directory ());
               reserved = TRUE;
             }
 
+          if (utf8_catalog_dir != NULL && *catalog_dir == NULL)
+            {
+              GError *error = NULL;
+
+              /* The passed-on catalog directory is in UTF-8 because this is
+               * usually hardcoded (in which case it's easier to request a
+               * specific encoding, chosen at development time, rather than the
+               * "OS encoding", depending on runtime).
+               * But now we want to transform it to the encoding used for
+               * filenames by GLib.
+               */
+              *catalog_dir = g_filename_from_utf8 (utf8_catalog_dir, -1, NULL, NULL, &error);
+
+              if (*catalog_dir == NULL)
+                g_printerr ("[%s] provided catalog directory is not proper UTF-8: %s\n",
+                            procedure_name, error ? error->message : "(N/A)");
+
+              g_clear_error (&error);
+            }
+
           if (*catalog_dir && ! reserved)
             {
               if (g_path_is_absolute (*catalog_dir))
@@ -982,6 +1001,8 @@ _gimp_plug_in_set_i18n (GimpPlugIn   *plug_in,
               *catalog_dir = g_file_get_path (plug_in->priv->translation_domain_path);
             }
         }
+
+      g_clear_pointer (&utf8_catalog_dir, g_free);
     }
 
   if (use_gettext && ! g_file_test (*catalog_dir, G_FILE_TEST_IS_DIR))
diff --git a/libgimp/gimpplugin.h b/libgimp/gimpplugin.h
index fd054be367..687062920b 100644
--- a/libgimp/gimpplugin.h
+++ b/libgimp/gimpplugin.h
@@ -134,8 +134,8 @@ struct _GimpPlugInClass
    * @gettext_domain: (out) (nullable): Gettext domain. If %NULL, it
    *                  defaults to the plug-in name as determined by the
    *                  directory the binary is called from.
-   * @catalog_dir:    (out) (nullable): relative path to a subdirectory
-   *                  of the plug-in folder containing the compiled
+   * @catalog_dir:    (out) (nullable) (type utf8): relative path to a
+   *                  subdirectory of the plug-in folder containing the compiled
    *                  Gettext message catalogs. If %NULL, it defaults to
    *                  "locale/".
    *
@@ -157,12 +157,12 @@ struct _GimpPlugInClass
    * simply set the method to %NULL, or possibly implement this method
    * to do something useful for your usage while returning %FALSE.
    *
-   * If you wish to tweak the @gettext_domain or the @localedir, return
-   * %TRUE and allocate appropriate @gettext_domain and/or @localedir
+   * If you wish to tweak the @gettext_domain or the @catalog_dir, return
+   * %TRUE and allocate appropriate @gettext_domain and/or @catalog_dir
    * (these use the default if set %NULL).
    *
-   * Note that @localedir must be a relative path, subdirectory of the
-   * directory of `gimp_get_progname()`.
+   * Note that @catalog_dir must be a relative path, encoded as UTF-8,
+   * subdirectory of the directory of `gimp_get_progname()`.
    * The domain names "gimp30-std-plug-ins", "gimp30-script-fu" and
    * "gimp30-python" are reserved and can only be used with a %NULL
    * @catalog_dir. These will use the translation catalogs installed for