"The voice in your head has an accent, you just can't hear it"
Everyone who thinks in a language thinks with the accent they speak. If you're British, your inner monologue has a British accent. If you're Australian, it's Australian. But here's the wild part: you don't notice it because it's just... your normal voice.
You can only recognize an accent as an "accent" when it's different from your own. Your accent is invisible to you—it's just "how people talk." But to everyone else, you have a distinct accent.
What about bilingual people? Do they switch accents when they switch languages in their head? Do they have multiple inner voices? The mind gets blown even further when you consider:
Here's another layer: can you have an accent in sign language? The answer is yes! Regional signs differ just like spoken accents. So even deaf people who think in sign language have a "visual accent" in their thoughts.
You've been listening to your inner voice your entire life, but you've never actually "heard" your accent. It's the most familiar voice you'll never truly hear objectively.