English equivalent for "When the head is present, tail should not wag."

Solution 1:

Something cool here could be:

"When I want your opinion, I'll tell it to you." - Indicating the speaker has power over the one they are talking too, doesn't care what they think, and he/she has spoken out of turn. Sometimes "I'll tell it to you." can be replaced with "I'll beat it out of you.", when the speaker is more violent.

"Know your role, and shut your mouth!" - Not so much an idiom as a wrestler's catch phrase. But it fits, and it is fun to say.

Solution 2:

One similar piece of advice we have is "respect your elders", although that's not the same as talking out of turn when some random person of higher status should be talking.

Aside from talking "out of turn" or "out of place", I'm not sure we have a good expression for that.

Laura thannode vaalai aattunnu. (I hope that means Laura wags her tail, in Malayalam.)

Solution 3:

Before putting the bullet between M's eyes, A could say that M should be put in his place.

to let someone know that they are not as important as they think they are

A could rhetorically ask M as to who's running the show, before pulling the trigger on M's head.

to be in charge; to be in command

[The Free Dictionary]

Note

I tend to concur with @FumbleFingers, I think the saying in Tamil/Malayalam is derived from tail wagging the dog. The saying in Tamil goes something like:

_{தல இருக்கும் போது வால் ஆட கூடாது {thala irukum pothu vaal aada koodathu}}

Solution 4:

Children should be seen and not heard

From The Phrase Finder

In the original form of this proverb it was specifically young women who were expected to keep quiet. This opinion is recorded in the 15th century collections of homilies written by an Augustinian clergyman called [John] Mirk's Festial, circa 1450:

Hyt ys old Englysch sawe: A mayde schuld be seen, but not herd.

A 'sawe', or 'saw' as we would spell it now, was a mediaeval term for saying or proverb. It has the same root as the words 'say' and 'saga'.

While the expression was aimed at women, the Old English names denoting gender are now somewhat altered. A 'mayde' was normally a young female, usually unmarried, although it was also used to denote celibate men. Girls however, could be of either sex, the term simply meaning young child.

If you used this phrase today, mothers everywhere (in the US at least) would rise up and denounce you. But how I wish the airlines would enforce it!