[i18n] Make gateway strings translateable (#9933)

See <https://www.mobileread.com/forums/showthread.php?p=4282773#post4282773>.

frontend/device/generic/device.lua

@@ -495,12 +495,12 @@ function Device:retrieveNetworkInfo()
                 pingok = os.execute("ping -q -w 3 -c 2 `ip r | grep default | tail -n 1 | cut -d ' ' -f 3` > /dev/null")
             end
             if pingok == 0 then
-                result = result .. "Gateway ping successful"
+                result = result .. _("Gateway ping successful")
             else
-                result = result .. "Gateway ping FAILED"
+                result = result .. _("Gateway ping FAILED")
             end
         else
-            result = result .. "No default gateway to ping"
+            result = result .. _("No default gateway to ping")
         end
         return result
     end