Lonsana Doumbouya - The Christian name is easy enough but I've mostly heard his surname pronounced by fans as 'Doom-Boy-Ah'. Now granted, it looks a bit like that to an English speaker, but I watched his game for Guinea on YouTube a few weeks back and it had French commentary. They pronounced his name 'Doom-Boo-Ya' which does sound more French.
Owain Fôn Williams - Now this is one that no-one seems to get it right at all. First of all I've heard some fans referring to him simply as 'Williams'. This is wrong and would be akin to calling someone with the surname MacDonald as 'Donald'. His first name is not pronounced like Owen or 'Oh-Wayne' as some seem to think. It is in-fact 'Oh-Ine' (Ine sounding like wine with the W taken away). His surname is also not pronounced 'fon', it is in-fact pronounced as 'Vorn'. I've also been told by a Welsh speaker that the Williams is not pronounced as it would be for a non-Welsh person, as in Welsh you actually acknowledge the A, meaning 'Will-Ee-Ams' is how to say it. Altogether, the correct pronounciation is 'Oh-Ine Vorn Will-Ee-Ams'.
I expect these to be pronounced correctly by all fans from now on.