"約定俗成"的英文?

A convention或
A conventional ...saying(約定俗成的說法/口語), practice(約定俗成的做法)

字典都會有這解釋用法，但小心全句寫作時有時會給人「固有」「一承不變」「慣常」之意。

假如不是用作形容詞，而是說某一東西是因為約定俗成而產生，可以考慮不太convention的寫法。

xxx is the result by the consensus of the mass.
這東西是由於約定俗成(由眾多人產生共識而成的)所致。
(例如：方言或語文)　

約定俗成

1 )Basic commitment.
2) Common commitment

Which means a commitment (or contract) is made base on a common rule
or a traditional rule.

It is a norm. OR
It is a convention. OR
It is a tradition. OR
It is a normal practice.

約定俗成:
=各物法則等為社會習用或公認.
=The names of objects and principles which have been used or accepted in the society.
亦可以用下面衣d.
1.established by the people through long social practice;
2.An agreement has become a practice.;
3.accepted due to customs;
4.accepted through common practice;
5.established by usage;
5.the common run of mankind