Part of what we think of as our hearing faculty is in our ears and part is in our brains.
We here all noise with our ears and then our brains do all kinds of filtering and analyses:
- At one level human speech sounds stand out because these sounds fall within a particular range. Our ears contain tiny hairs of different lengths which vibrate in sympathy with different sound frequencies. The vibrations are converted into electrical signals and sent to the brain. This is very much like Fourier analysis.
- At a much deeper level, languages we know very well stand out more because our brain subconsciously processes the sounds as speech.
This is where ambient language can enter your thoughts and confuse you. For me my Spanish is sub-fluent so mostly I can concentrate on a conversation I'm having with people without bothered by a loud TV in the background which would interfere with my thoughts in English. I do however find that I subconsciously eavesdrop on people speaking Spanish, even when I haven't used it for a while.