begin transition from English to ISO-639

This patch converts all English language strings used within
depressionbot's localisation interface (that is, strings.json) to their
ISO-639-1 equivalents.

The purpose of this patch is to make the strings.json file less
English-dependent.

All languages with an ISO-639-1 code have been converted; that is,
English, Spanish and Welsh, to ``en``, ``es`` and ``cy`` respectively.

This patch does not attempt to force a solution to the issue of Na'vi:
the language does not have a large enough corpus to warrant its own ISO
639-3 code, and as such there is no perfect solution. For the time
being, this patch keeps the Na'vi language as its English language
string ("Na'vi"). A possible solution to this discrepancy includes using
the ISO 639-3 code ``art``, used for artificial languages that do not
qualify for an official code, or by using local use codes specified
in ISO 639-3 (``qaa`` to ``qtz``).

This patch requires collaboration with upstream repositories that also
use strings.json, such as the Github and Stats modules.

Initial fix for #234.