InstantTranslate: speedup by resources list browser-side caching

Problem

Currently, list of available language resources per source and target language pair is reloaded each time source text is changed, and this increases overall response time

Solution

List of available langauge resources for currently selected source and target languages - should be loaded once and then cached on browser-side, to be further used with no need to do such a preliminary request again each time source text is changed. This should work that way unless 'Detect language' is selected as source language.