Fixed merge error in MemberTest
API JS i18n files with short locale names, generated by build task

The files are uploaded to transifex.com now, which means we needed
to consolidate the file names (= locales). Since Transifex doesn't allow
arbitrary JavaScript structures, we're using an intermediary JSON
format which gets transformed into the JS used by the framework.

See https://github.com/silverstripe/silverstripe-buildtools/commit/b59f3858afd5c46219a42b04b2742c7d247199c9 for generator logic.

Fixes https://github.com/silverstripe/silverstripe-framework/issues/2499