I know some time has passed since the last reply to this thread but i just found it through google so others might too.
I just ran into the same issue. What solved it for me was transaction I18N.
Under I18N System Configuration (Report RSCPINST) only EN was listed as a language, but the profile stated DE as default. I added DE, activated and the selection of the language for crm_ui_start worked.
(Though I didn't have to set that, crm_ui_start just worked with the default setting in german after that).
Hope it helps.
Best Regards,
Paul