I suppose, the title suffice as my question, however I'd like to add here that, even if so they are considered informal;
Why is that? and who determines whether a word is formal or informal?
I suppose, the title suffice as my question, however I'd like to add here that, even if so they are considered informal;
Why is that? and who determines whether a word is formal or informal?
The terms formal and informal refer to how social norms affect word usage. It is a matter of cultural convention (within a given group) that determines whether a word is formal or informal. It's a bit like formal and informal clothing: it varies between cultural groups and changes over time. In other words, usage determines whether a word is formal.
The authority on whether a word is formal would be the community of sociolinguists, linguistic anthropologists, and lexicographers.
The question of why a word is informal is a difficult one and arguably has more to do with social factors than with the language itself. For example, in many former colonies, the (usually European) colonizers' language is the formal language used in government and academia, while the local dialect is considered informal and unsophisticated. It's an interesting area of inquiry, but outside the scope of ELL.