How can i install the greek language?
You'll need KEYB and the appropriate codepage (??) from CPIDOS and DISPLAY, MODE. Kernel 2039 ("unstable") has some COUNTRY.SYS support; otherwise you'd have to use older 2037. (But note that most tools probably don't provide Greek NLS translations. Feel free to contribute some!)
If you just want to edit Greek text, esp. in Unicode, you can use Blocek, Mined, GNU Emacs, or similar (JED or VILE somewhat).
Here's some links:
For just displaying on your screen the appropriate fonts (EGA, VGA, or newer), you need these:
Apparently Greek codepages 737 and 869 are in EGA5.CPX.
For entering Greek letters inside most regular DOS apps (without special support built-in individually), you need these:
(Note that there seems to be a small incompatibility bug in latest 2.6, so just use kpdos25x.zip for now.)
Seems Greece layout support is in KEYBRD2.SYS.
NLSFUNC is for uppercase (collate) tables, y/n, currency, decimal separator, etc. It uses COUNTRY.SYS (found with appropriate kernel, e.g. 2039 or older 2037).
If you just want to edit multilingual text (Unicode or otherwise), Mined is a pretty good choice. It does support some nifty features, including now (2000.16) various inputs including Greek. It will auto-map your UTF-8 text to cp737 if you've set it up correctly via MODE first and start via "mined -u greek.txt". (I had a screenshot, but I can't seem to attach it here, argh.) I have an older (2000.14, mostly complete) 16-bit DOS (Turbo C) build if you want it. Not sure if the latest still builds like that (even then it needed a few patches), but I'm sure the author (TW) would be happy to help if desired.
I made a (not yet published) floppy disk for you to show you all these things if you want it. Otherwise, you'll have to set it up yourself:
REM FreeDOS display is not a device driver
mode con cp prep=((737 869) a:\fdos\ega5.cpx)
mode con cp sel=737
REM mode con cp sel=869
(I hope this is all correct, it mostly works for me, but since I'm unfamiliar with Greek or using it, I can only truly make an educated guess here.)