Learn English – Difference between which and what

"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.

BE is notoriously difficult to define, because you can't say what it "is" without in effect using the word itself.

But the difference between BE and BECOME is relatively easy.

• BE designates a state, something which continues unchanged through time.
• BECOME designates an event, a change of state.

Right now you are (BE) not yet a software developer. You are perhaps a student of software development. That is a state. You will continue to BE a student until your state changes. At that point you will BECOME a software developer. After that point you will no longer BE a student, you will BE a developer.

So both ways of describing your ambition are correct. You want to BE a developer; but in order to achieve that state you must BECOME a developer, you must change your present state.