for using xmm files (xmodmap) 
 $ xmodmap filename
say
 $ xmodmap inscript.hi.map
to get default keymap
 $ xmodmap default

Chek in keymap files for a line
 keycode <someno> = ModeSwitch  XF86ModeLock

if XF86ModeLock is present after ModeSwitch , this basically locks the
mode (u need not keep pressing the modifier key), it works like a
toggle. But for this  in XF86Config file u need to disable XKB by
giving   XkbDisable  (this was with old XFree86, I will let you know
what to do with new X-4.0) 

> I ahve used mode_switch for some key.
> But whenever i press down that particular key (which i defined as
> mode_swirtch ) the key is chaged toi the defiend lanf\guage.
> how to lock that particular key in a particular case.
>
Thats with Xmodmap keymaps. To be able to use it without keeping the
key pressed you need to modify the XF86Config / XF86Config-4 file to
have in InputDevice section of Keyboard have the option XkbDisable . 

Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "keyboard"
        Option      "XkbDisable"    
EndSection

 To use xkb files, do the following
 copy the xkb files to /usr/X11R6/lib/X11/xkb/symbols/

  # cp inscript.hi.xkb  /usr/X11R6/lib/X11/xkb/symbols/hi
 (do same for other languages also)

 to load keymap 
  $ setxkbmap -layout hi -option grp:ctrl_shift_toggle

 use CTRL+SHIFT  toggle
  
  $ setxkbmap -layout hi -option grp:caps
 lets you toggle with CAPS lock (but caps lock definition changes for
current X session, donno yet how to get it back).

or to set is as default always then add the option  XkbLayout

Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "keyboard"
        Option      "XkbLayout" "hi"
EndSection


Writing your own keymaps
------------------------
Writing a for use with xmodmap
best resource is man keymaps(5),
for the man page