Open the file controllers.ini in the main DW folder with any text editor (Windows notepad just works fine) and scroll down until you see this:
[Interface] "FFG" 12
[BitmapName] "FFG_US.jpg"
[DLL] "Bridge.DLL" "X000Y000.bmp" 569 408
;/// soundEngineHum sndtype (none = 0, full=1, full no os =2, muted=3, muted no ownship=4) truthmode
[DLLSOUND] 1 0 0
[DLL] "AcousticDisp.DLL" "X035Y000.bmp" 570 409
[DLL] "ASTAC.DLL" "X000Y035.bmp" 571 410
[DLL] "Active.DLL" "x035y035.bmp" 572 411
[Nav] "X000Y175.bmp" 416
[DLL] "TowedArray.DLL" "x035y070.bmp" 573 413
[DLL] "EW.DLL" "X000Y070.bmp" 574 412
[DLL] "Radar.DLL" "X035Y105.bmp" 575 415
[DLL] "TMA.DLL" "X000Y105.bmp" 576 414
[DLL] "WeaponsControl.DLL" "x035y140.bmp" 577 417
[DLL] "FloatingCamera.dll" "NOLINK"
[DLLPOPUP] 1 0 0 285 230
[DLL] "TorpedoControl.DLL" "x000y140.bmp" 578 418
[DLL] "MachineGun.DLL" "x035y175.bmp" 579 419
;/// soundEngineHum sndtype (none = 0, full=1, full no os =2, muted=3, muted no ownship=4) truthmode
[DLLSOUND] 1 2 1
[TEXTDLL] "Text"
[RECO] "VoiceReco.DLL"
[Color] "255,140,0"
[Color] "25,30,50"
[Color] "40,50,75"
[Color] "0,255,0"
[Color] "145,75,0"
[Color] "115,105,105"
[Color] "255,140,0"
[Color] "255,140,0"
[Color] "0,255,0"
[Color] "80,80,80"
[Color] "255,255,220"
[Color] "0,0,0"
[Units] "0"
[MenuImage] "FFG7_menu.bmp"
[MenuPos] "-9.3,0.3,-99.0"
[END]
The text I have highlighted in red tells the game wether to use imperial or metric units. Change it to "1" instead of "0" and you should have now metric units in all displays.
BTW you can do this with any playable unit, just look for the relevant line in the block dedicated to that unit and change it to "1".