The base language must match for the installation.
Just came across this presentation that I think might help add some clarity.
•Titan supports multi-lingual CRM deployment
•Multi-lingual configuration rules
–CRM
•Base language of CRM server and CRM client has to match
–Server
•Server Stack has to be all in the same language
•Server Stack has to either match the CRM Server base language, or be English
–Client
•Client Stack does not have to match the CRM Client base language
•Client Stack can be in different languages in a single deployment
http://blogs.msdn.com/midatlanticcrm/attachment/7212865.ashx