I have Russian and US English locales. When I'm typing a text in English, it gets underlined until I press Space or some other special keys. When it's underlined, it's kind of invisible to the application where I'm typing it in. For example, in Sublime Text, it isn't even displayed until I press Space. In a browser it won't suggest me a URL until I press Space. It kind of gets buffered — saved in a temporary buffer and then released. And other bizarre stuff like this.
This doesn't happen always, sometimes, around 10% of the time, it works properly, I don't know what it depends on. And it doesn't happen, as far as I can remember, with Russian at all.
It's Arch, XFCE. I've done a bit of a search and haven't found anything anything related to this behaviour.