You can start XChat-WDK in whichever language you want (for which L10n exists). You can use the regional settings of Windows, or set the LC_ALL user environmental variable. The value of the variable must be the two letter country code for your country. If in doubt, have a look at the share\locale folder. You have to restart XChat-WDK for the changes to apply.
You may also use a batch file to make the setting apply only for XChat-WDK:
This sets the language to English. You may use fr for French, de for German, etc. Save the code above as run.bat, and copy it to the XChat-WDK install folder. You can then start XChat-WDK in the desired language by running the batch file.