The configuration file setting
com.openexchange.guard.defaultLanguage will be added to the configuration file. This value will be used for the default language if not specified by the client

Added

# Default Language

# Set to fallback language if not specified by the client. Will be
# used for the templates sent out. Please make sure the translations are
# present in the l10n directory

# com.openexchange.guard.defaultLanguage=en_US