Which system files are safe to delete? I got only 160MB internal storage out of which 100MB is reserved by android operating system.  

Which System files and folders are safe to delete?

For example:  


de, fr, es and it files in /system/tts/lang_pico/  seems useless  
/cust/hw/es folder seems uselss  
As I have moved many of my application to SD Card, many folders in /mnt/asec seems useless  


So which files/folders of Android system are safe to delete so that I can make more space for applications
Not a description of all system files that can be removed but the Cyanogen Barebones page gives a pretty good description of which system applications can be removed and how. This may help you clear out some space.
Details here:
http://oldwiki.cyanogenmod.org/index.php?title=Barebones