How can I get my GF to talk more like a grown-up?

I love her to death, but she talks in a combination of baby-talk, lol-speak, and broken English most of the time (she's a native English speaker). It can often be cute, but what started as an endearing thing has quickly morphed into how she talks to me 95% of the time.

She didn't talk like this when we met. It's a recent development.

It's hard to get in the bedroom mood when she sounds five all the time.

So how can I fix this? I've tried talking to her about it, and she'll stop for a few days, but then it starts back up again!