It isn’t offensive for a non-Scot to wear a kilt – we’d probably be quite pleased you’d made the effort!
However as others have said, it wouldn’t be good if you wore one of the military tartans cos that might well be offensive to quite a few folk, especially if they lost a family member wearing that tartan.
There are other things to consider though, and these are just as important. Do you have the legs for a kilt? And do you have a large belly? I don’t mean to be “fattest” but a big belly in a kilt, or one hanging over it does nothing for you and will make us Scots smirk a little. Thats not to say no Scottish Man has a big belly, but they tend not to wear kilts in that case!
And your legs are important too, cos you need to have muscular calves to look right in a kilt. And get the length right for a man! If you need to see how it should look, see Jamie, the highlander in Outlander. Now he is what a Scot wearing a kilt should look like! ;-}
My father who is Lancastrian, wouldn’t wear a kilt to my wedding because he isn’t a Scot and he felt it was wrong to do so. And I applaud him for it! He feels that you should only wear a kilt if you do have some Sottish blood in your veins. He does but its pretty diluted after many generations and some Irish in there too!
The tartans are pretty important up here in the Highlands too cos believe it or not we will recognise which one you have on and ask you about your ancestry. I’m a MacGregor and know that tartan anywhere. Also Royal Stuart, although its popular maybe isn’t a good choice. Its a wee bit over used and a bit NAFF, and it does belong to Royal Blood!
If you really want to wear a kilt you could use a non tartan pattern type material and they are pretty cool looking. Specially if its in a dark Grey or a earthy green colour – that makes for a pretty nice looking man in a kilt!