Learn English – Difference between “speak of” and “speak about”

"One in ten" is a ratio (10%) but does not define the size of the sample or group.

The definition you have for "out of" is correct.

While your phrase "One out of ten people speaks (English) as their mother tongue" is technically OK (though it should be speaks since "one" is singular), it makes no sense because we know more than 10 people speak English. You would need to add a qualifier, such as:

"One out of ten people in my office speaks (English) as their mother tongue".

It implies there are only 10 people in your office.

Sometimes, not only in English but in all languages, we want to emphasize certain situations. And then the language itself gives us devices that aren't always common, but we use them anyways, based on known and meaningful expressions.

I am very hungry

We can depict that the subject has surpassed the status of just "hungry" for they must have stayed a long time without eating. This is a known meaningful expression.

Murder is very illegal

From this, even if it doesn't make much sense in the binary nature of the word "legal", we can depict that "murder" is a crime that, morally or ethically, has surpassed the status of "illegal".

We can state that by looking at another not-so-serious crime:

Parking on the sidewalk is illegal

Yeah, we all know it is illegal and wrong. But it is a petty crime compared to murder. In some countries murder is penalized with life imprisonment, even with death penalty, while parking on the sidewalk gives you a fine and, in the worst of the cases, your car is towed.

We can still say that "Murder is illegal", of course it is, but in the sentence, the "very illegal" was made to emphasize.