Dealing with characters not supported by a font

I have a website which is used by people all around the world. I use a certain font which only just came to my attention does not support characters which my users have mentioned such as ğ, ş and ı.

As the font I use across my website does not have correct styling for these characters they look a bit odd. While this is not an issue for most users, there are a minority who this may be an issue for.

What could I do to make foreign characters appear more streamlined? Thanks.