GenoPro is a piece of software that allows you to create quite complex family trees. Changing it to Gaelic is easy, after downloading it, all you have to do is go to Language » Scots Gaelic. GenoPro is a commercial product but it’s not hugely expensive and works really well – and of course it’s in Gaelic!
It also works on Macs if you have sofware that let’s you run Windows programs on it. You can get more information on how to do this on GenoPro’s website.