Fixes unit-prefork in online for me.

Kendy suggested to also change the forced locale to en-US.UTF-8
instead of just en-US, but that caused:
warn:i18nlangtag:32416:1:i18nlangtag/source/languagetag/languagetag.cxx:1369:
LanguageTagImpl::convertLocaleToLang: with bAllowOnTheFlyID invalid
'en-US.UTF-8'.

This reverts commit c3d25b8b.

Change-Id: I2cd8c50a783918c380505e7a3aaf37d578e76031
Reviewed-on: https://gerrit.libreoffice.org/31489Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>