Proposing core translations only when a xx-XX.xml file is present in the xx-XX folder + using one language and one client only in translations view to cope with memory issues

replaces https://github.com/joomla-projects/com_localise/pull/190 which was merged per error and then reverted

See there for instructions.

# DO NOT MERGE until the filters issue is fixed

https://github.com/joomla-projects/com_localise/pull/190#issuecomment-52289516