Does this message appear directly after the startup sound and the Happy Mac icon? Would it be correct to assume that the Desktop (with a Wastebasket icon) is not shown at all? Any difference if you hold down the Shift key after startup?
A bootable system floppy would be of help. A plain Macintosh SE uses 800K DD diskettes (the FDHD model can handle 1.44 MB HD disks as well). Disk images can be downloaded from Apple, but you would need another older Mac (with a built-in floppy drive and the Disk Copy program) to create properly sector-copied floppies. If necessary, try to find a User Group in your area.
In English, this is the screen message. "SYSTEM ERROR. COVER THE BATTERY AND APPLICATION" I'm not familiar with this error message & what it means.
Since the SE has not been pluged into AC power for a long time, the internal PRAM battery has surely discharged & needs replacement. This link http://lowendmac.com/compact/macintosh-se30.html has info on the SE & a link for a replacement battery
You can get replacement system floppies at this site. http://rescuemyclassicmac.com/
The first place to start looking is for any external SCSI device, especially a hard drive or zip drive. If you can boot your computer from an external device, you can restore your system and hard drive. Are you in Europe or Quebec? There is a mac user group in Montreal - http://www.techniconseils.ca/en/portfolio-lcmm.php and also in Toronto - http://www.logicbbs.org/ (They go back to the Apple //).