“No one can be too talkative without often saying something that makes him look foolish, for the wise man's saying goes: "Whoever talks too much does himself a bad turn."”

"1901", p. 76. Sometimes misquoted as "If fifty million people say a foolish thing, it is still a foolish thing". Sometimes misattributed to Bertrand Russell or Anatole France
A Writer's Notebook (1946)

W. Somerset Maugham, A Writer's Notebook (1949), entry for 1901
Sometimes misquoted as "If fifty million people say a foolish thing, it is still a foolish thing."
Sometimes misattributed to Anatole France
Note that Russell does say something similar in Marriage and Morals (1929): "The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible."
Misattributed

Original: (pt) Quem fala demais dá "bom-dia" a cavalo!

Unidentified page
A Writer's Notebook (1946)

Source: Charmed Thirds

G 32
Aphorisms (1765-1799), Notebook G (1779-1783)

As quoted in NEA Journal : The Journal of the National Education Association‎ Vol. 41 (1952) p. 300
Context: One of the basic causes for all the trouble in the world today is that people talk too much and think too little. They act too impulsively without thinking. I am not advocating in the slightest that we become mutes with our voices stilled because of fear of criticism of what we might say. That is moral cowardice. And moral cowardice that keeps us from speaking our minds is as dangerous to this country as irresponsible talk. The right way is not always the popular and easy way. Standing for right when it is unpopular is a true test of moral character. The importance of individual thinking to the preservation of our democracy and our freedom cannot be overemphasized. The broader sense of the concept of your role in the defense of democracy is that of the citizen doing his most for the preservation of democracy and peace by independent thinking, making that thinking articulate by translating it into action at the ballot boxes, in the forums, and in everyday life, and being constructive and positive in that thinking and articulation. The most precious thing that democracy gives to us is freedom. You and I cannot escape the fact that the ultimate responsibility for freedom is personal. Our freedoms today are not so much in danger because people are consciously trying to take them away from us as they are in danger because we forget to use them. Freedom unexercised may be freedom forfeited. The preservation of freedom is in the hands of the people themselves — not of the government.

Often attributed to Plato, it cannot be found in any of his writings ( see this http://languagelog.ldc.upenn.edu/nll/?p=796). The quote is attributed to Plato in A Dictionary of Thoughts: Being a Cyclopedia of Laconic Quotations from the Best Authors of the World, Both Ancient and Modern (page 560) by Tryon Edwards.
Misattributed

Stanza 1.
The Second Jungle Book (1895), If— (1896)
Source: If: A Father's Advice to His Son
Context: If you can keep your head when all about you
Are losing theirs and blaming it on you,
If you can trust yourself when all men doubt you,
But make allowance for their doubting too;
If you can wait and not be tired by waiting,
Or being lied about, don't deal in lies,
Or being hated, don't give way to hating,
And yet don't look too good, nor talk too wise.