Admin Control Panel > Appearance > Import a Language
If you would like to return us a favor for translating this, please make suggestions for incorrect phrases, so we could fix it on our board as well.
Take in account that user_upgrades phrases have been modified for our needs, so you would need to change these in order to fit your board's theme/subject.
In addition, search for website.co.il and edit it (basically forum's terms and 404 pages).
Take in account that ACP was partially translated. I would NOT recommend translating it since it would be difficult to follow instructions for troubleshooting/add-ons configurations.