[Touch Dual, MDA Touch Plus ROM Development] d_train:
I don't speak Spanish, so I am not sure I can help.

You need to get the tools kiliss mentioned htcRIE_0.5.0.12.zip so that you can see the mui files that you need (here is a list of the ones when I opened one of the files)

AdvancedNetwork.exe.0409.mui
AGPSSettings.exe.0409.mui
AudioBooster.exe.0409.mui
AudioManager.exe.0409.mui
bbcbres.dll.0409.mui
BluetoothSetting.dll.0409.mui
browsres.dll.0409.mui
Camera.exe.0409.mui
ClearStorage.exe.0409.mui
CMBandSwitching.exe.0409.mui
CMBluetooth.exe.0409.mui
cmhomeres.dll.0409.mui
CMPhone.exe.0409.mui
CMPin.exe.0409.mui
commctrl.dll.0409.mui
commdlg.dll.0409.mui
CommManager.exe.0409.MUI
ConnectionSetup.cpl.0409.mui
ConnectionSetup.exe.0409.mui
ConnectionSetupAuto.exe.0409.mui
ConnSetupDload.dll.0409.mui
coredll.dll.0409.mui
coreres.dll.0409.mui
crypt32.dll.0409.mui
CUIHandler.dll.0409.mui
DeviceInfo.exe.0409.mui
EmailSetupWizard.exe.0409.mui
fatutil.dll.0409.mui
filesys.exe.0409.mui
flashresource.dll.0409.mui
getMMSCtxMenuInfo.dll.0409.mui
GPSPrivacy.dll.0409.mui
gwes.exe.0409.mui
HTCAlbum.exe.0409.mui
HTCApplication.dll.0409.mui
HTCGeoTOS.dll.0409.mui
HTCMessage.exe.0409.mui
HTCMsgEnhance.exe.0409.mui
HTCMsgEnhanceDLL.dll.0409.mui
HTCPhotoPicker.exe.0409.mui
HTCSettings.dll.0409.mui
HTCSoundSettings.exe.0409.mui
HTCUSBToPCPopUp.exe.0409.mui
HTCVolumeControl.dll.0409.mui
HTC_HDlg.exe.0409.mui
IA_Zip.exe.0409.mui
ieceext.dll.0409.mui
ipsecvpnres.dll.0409.mui
JBlend_AMS.exe.0409.mui
JBlend_VM.exe.0409.mui
jscript.dll.0409.mui
LongPressEndKeyRes.dll.0409.mui
Manila2D.exe.0409.mui
Manila2DListLauncher.exe.0409.mui
MdsRes_wpc.exe.0409.mui
MicrophoneAGC.exe.0409.mui
miscres.dll.0409.mui
mlang.dll.0409.mui
MMSTransport.dll.0409.mui
netmui.dll.0409.mui
NotificationLayout.dll.0409.mui
NotificationManager.dll.0409.mui
notify.dll.0409.mui
officeres.dll.0409.mui
OutlookLiveUpdaterResources.dll.0409.mui
outres.dll.0409.mui
phcanrc.dll.0804.mui
Powerexe.exe.0409.mui
PowerOffWarning.exe.0409.mui
pptres.dll.0409.mui
quartz.dll.0409.mui
Recorder.exe.0409.mui
SetSysTimeZoneImpl.exe.0409.mui
SettingImprovement.exe.0409.mui
shdoclc.dll.0409.mui
shellres.dll.0409.mui
shellresapps.dll.0409.mui
shlwapi.dll.0409.mui
SIM_MGR.exe.0409.mui
SMSInboxThreading.exe.0409.mui
STK_UIPPC.exe.0409.mui
StorageLow.exe.0409.mui
StreamingPlayer.exe.0409.mui
syncres.dll.0409.mui
tapres.dll.0409.mui
TaskBarIconMgr.dll.0409.mui
TFSettings.exe.0409.mui
tshres.dll.0409.mui
unimodem.dll.0409.mui
urlmon.dll.0409.mui
USSD_Service.dll.0409.mui
waveapi.dll.0409.mui
wininet.dll.0409.mui
WMMarketplaceFullClient.exe.0409.mui
wsp.dll.0409.mui

These can be found in one of three places - a stock HTC Spanish Nike ROM, a stock HTC Spanish MEGA ROM and the SYS language files for the particular version you are wanting to translate (so the Spanish language files for SYS 21911 or 23569 in this case)

SYS - http://xdaforums.com/showthread.php?t=650783

MEGA - http://shipped-roms.com/shipped/Meg...Radio_Signed_61.41t.25.31_3.48.25.27_Ship.exe

Nike - http://shipped-roms.com/shipped/Nik...412.1_Radio_NikeMR_1.65.28.25_signed_Ship.exe

These are the starting point, without them you can't perform the remaining steps kiliss mentions.

You will need to extract the shipped ROMs - using something like Ervius Visual Kitchen (search the forum for the link to the thread).

DT