That was right! I sniffed it out and fixed it. I'll edit this report appropriately.
That's not the same issue either! This is only to do with the default font, which breaks when you use a custom language pack. (Not a default-shaped font, just default.)
I think Kumasasa is right and this is related to MC-2488, but it's not to do with using a custom font.
That's a different issue! That issue is about custom fonts displaying incorrectly, whereas this is about the default Minecraft font not showing up properly when using a language pack.
They still work fine, you just need to rename the "sound" folder to "sounds". It's a pain, I know.