I have read in many places on the Internet that "yeah" is considered unprofessional. And that using "yes" is preferred and considered more formal. What is the reason behind it?
possible duplicate of When is it appropriate to use "Yeah" and "Yep" as variants of the word "Yes"?– virmaiorFeb 9, 2014 at 9:56
Doesn't your own language have an equivalent meaning in ways to say yes?– LambieJun 22, 2018 at 17:01
Because it is informal and colloquial.
yeah adv. Informal Yes.
yeah American English, colloquial, by 1863, from drawling pronunciation of yes.
You can check this similar question as well:
When is it appropriate to use "Yeah" and "Yep" as variants of the word "Yes"?
Because "yeah" is an informal and almost lazy way of saying yes. In yeah,you are not pronouncing the word yes correctly.