[fix, i18n] Auto-save book metadata: fix interval string (#5729)

See https://github.com/koreader/koreader/pull/5687#issuecomment-570226614 and followups.

frontend/ui/elements/common_settings_menu_table.lua

@@ -7,6 +7,7 @@ local NetworkMgr = require("ui/network/manager")
 local UIManager = require("ui/uimanager")
 local TimeWidget = require("ui/widget/timewidget")
 local _ = require("gettext")
+local N_ = _.ngettext
 local Screen = Device.screen
 local T = require("ffi/util").template
 
@@ -372,7 +373,7 @@ local function genAutoSaveMenuItem(value)
     if not value then
         text = _("Only on close, suspend and exit")
     else
-        text = T(_("Every %1 mn"), value)
+        text = T(N_("Every minute", "Every %1 minutes", value), value)
     end
     return {
         text = text,
@@ -396,7 +397,7 @@ common_settings.document = {
                 if interval == false then
                     s_interval = "only on close"
                 else
-                    s_interval = T(_("every %1 mn"), interval)
+                    s_interval = T(N_("every 1 m", "every %1 m", interval), interval)
                 end
                 return T(_("Auto-save book metadata: %1"), s_interval)
             end,