fix #170: Empty field in preferences. Also add a bad German translation as a placeholder

pd/nw/locales/de/translation.json

@@ -364,6 +364,8 @@
       "sr_tt": "Anzahl von Samples pro Sekunde",
       "advance": "Verzögerung",
       "advance_tt": "Verzögerung (msec)",
+      "callback": "callback dinge",
+      "callback_tt": "benutzen der callback API von Pd für diesen Audio-Backend",
       "blocksize": "Blockgröße",
       "blocksize_tt": "Anzahl von Samples in einem Block, der an die Audio-API gesendet oder von dort empfangen wird",
       "channels": "Kanäle",

pd/nw/locales/en/translation.json

@@ -366,6 +366,8 @@
       "advance_tt": "delay",
       "blocksize": "blocksize",
       "blocksize_tt": "number of samples in a block to be delivered to or received from the audio api",
+      "callback": "use callbacks",
+      "callback_tt": "use Pd's callback interface for this API",
       "channels": "channels",
       "channels_tt": "number of channels for this device",
       "input_title": "Input Devices",