Of what benefit is english language to a practise engineer an a scientist.

English is a versatile language. It also has one of the richest [if not the sole richest] technical vocabulary in all of human history. English is also the 'mother tongue' (if you will) of many well-known, international standards. Of course, other languages can develop a technically rich vocabulary. However, in terms of scope and usability, English has long since established itself as more or less the better tool for the job (for good reason). It's no wonder engineers and scientists use it in technical discussions, even if they share a different native language.

If you are in an English speaking nation, or if you often have to make presentations in the English language if you travel abroad a lot, English can come in handy. It is the language of commerce after all.

I'm not a fan of the English language, however, I do find myself using it on a consistent basis to communicate my Engineering and Technical points to others.