Learn Turkish: Onların kültürüne göre bu gelenek biraz tuhaf. - Compared to their culture, this tradition is a little weird.

The root is kültür (culture). We add -ü for the possessive (their culture, linking to onların). Finally, the word göre always requires the dative case, which is the -e at the end. Because we are adding a case ending to a 3rd person possessive, we must use the buffer letter -n- between them. So: kültür + ü + n + e.

Does göre mean 'according to' or 'compared to' in this sentence?

Here it means 'compared to', but göre can mean both depending on the context! For example, bana göre usually means 'according to me' or 'in my opinion'. But when you are evaluating one thing against another (like this tradition against their culture), it translates to 'compared to'.

Why isn't there a suffix on tuhaf to mean 'is'?

In Turkish, the 'is' suffix for the third person singular (-dir or -tir) is usually dropped in everyday language. You could write tuhaftır, but just saying tuhaf perfectly means 'is weird' all on its own.

What if I didn't include the word onların? Would it still mean the same thing?

Yes! Because kültürüne already has the 3rd person possessive suffix (-ü), the 'their' is built right into the word. Saying Kültürüne göre... still means 'Compared to their culture...' (or 'his/her culture,' depending on context).