Translations:Translation for developers/2/en

This synchronisation with Translatewiki needs three types of operation:
 * Add a string to Kiwix and Translatewiki, ie. introducing a dynamic string which will be replaced by the corresponding translation at the runtime.
 * Update the Kiwix strings with the translations coming from Translatewiki.
 * Add a new Translatewiki full translated language to Kiwix.