As a European, I do appreciate Legal as a format.
It just happens slightly too often that an A4 is not long enough and the last bit, including the signature, goes to the next page.
I wish Legal was common here. Or perhaps we could get a Long A4 with a third extra height or so.
The Letter format should just be replaced by A4 though.
As your links explain, C series is used for envelopes, while B and elongated A are special case that aren’t commonly used.
In any case, none of those series has an equivalent to American Letter.
The only paper that you will commonly find in European offices are A4 and (to a lesser extent) A3.
A big office printer might have 4 trays stocked with A4 and one tray with A3, for example.