Or know someone who doesn't sound like a native speaker of a language, even when they're fluent?

This is why you have a different accent in a foreign language.

The first reason is that sounds differ from language to language.

Open a textbook, and one of the first things you'll see is an alphabet,

explaining, for example, that the P in French is very different from the P in English.

Hold the palm of your hand up in front of your mouth and say Paris in English, you'll feel a little puff of air on your hand.

Now, try the same thing again, but try to remove that puff of air and you'll get something closer to the French sound.

Learning about letters individually tells you very little about how they work when they come together.

For example, when certain consonant combinations are forbidden in your native language,

you'll often find yourself inserting sounds to make them pronounceable in a foreign one.

The reason a Spaniard might tell you that he is from ehSpain is because SP is just not possible at the beginning of Spanish words.

It's the same reason that when English commentators talk about the French footballer, Kylian Mbappé,

they often find themselves inserting a third syllable to make it more pronounceable.

It's because in English, starting a word with an M and a B next to each other is just not done.

So people find themselves subconsciously adapting words of a foreign language to fit the rules of their own.

The next reason is stress.

No, not that kind.

It's where people put the emphasis on words.

For example, all French words are stressed on the final syllable, usually with a higher pitch and greater volume.

That's why French speakers will call the Texan City, yooston, instead of the English, Houston.

Meanwhile, English words often have a secondary stress in addition to the primary one.

Civilization, for example.

Ditching your languages stress pattern is often very hard and is not always well taught.

Other elements of a language's rhythm can be hard to pin down.

Cantonese and Italian, for example, are syllable timed, which means that every syllable has roughly the same duration.

If you repeat this sentence with every syllable roughly equally timed, you may find yourself halfway to imitating an Italian.

Whereas in English, the stressed syllables come at roughly regular intervals and the remainder are less distinctly pronounced.

This is how you can distinguish Italian from English being spoken through a wall, even if you can't make out any individual words or sounds.

If pronunciation, stress and rhythm were better taught,

you might not find yourself speaking like this next time you go on holiday.