Show titles in TOC as configured in manager/languages

A very few modifications in core to get more control about how language is shown in TOC in multilang journals.
The aim of the modifications was commented here:
http://pkp.sfu.ca/support/forum/viewtopic.php?f=9&t=13021&p=49967
The new feature also requires modifications submitted to "lib-pkp" submodule.

A deep review of the code (and specially of the literals in locale/en_US/manager.xml) is encouraged.