Switch i18n default language from Czech to English

[slaweet]

msgids in *.po and template files should be in English to allow l10n to other languages (Polish, Hungarian, ...) by translators who don't know Czech).

Has to be done for server side:
https://github.com/adaptive-learning/anatomy/tree/master/conf/locale
https://github.com/adaptive-learning/anatomy/tree/master/anatomy/templates

As well as for client side:
https://github.com/adaptive-learning/anatomy/tree/master/anatomy/static/po
https://github.com/adaptive-learning/anatomy/tree/master/anatomy/static/tpl

It would be nice to have a script for it.