When a sensible man
has a good cause to defend, to be eloquent
is no great feat. Your tongue is so nimble
one might think you had some sense, but your words
contain none at all. The powerful man
who matches insolence with glibness is worst than a fool.
He is a public danger!

[ὅταν λάβῃ τις τῶν λόγων ἀνὴρ σοφὸς
καλὰς ἀφορμάς, οὐ μέγ᾽ ἔργον εὖ λέγειν:
σὺ δ᾽ εὔτροχον μὲν γλῶσσαν ὡς φρονῶν ἔχεις,
ἐν τοῖς λόγοισι δ᾽ οὐκ ἔνεισί σοι φρένες.
θράσει δὲ δυνατὸς καὶ λέγειν οἷός τ᾽ ἀνὴρ
κακὸς πολίτης γίγνεται νοῦν οὐκ ἔχων.]

Euripides (485?-406? BC) Greek tragic dramatist
Bacchæ [Βάκχαι], l. 266ff [Tiresias/Τειρεσίας] (405 BC) [tr. Cacoyannis (1982)]
    (Source)

To Pentheus. (Source (Greek)). Alternate translations:

When the wise man hath found a specious topic
On which to argue, he with ease may frame
An eloquent harangue. Your tongue indeed
Is voluble like theirs who reason well,
But in your language no discretion reigns.
He who possesses courage, sovereign power. A
And fluency of speech, if not endued
With wisdom, is an evil citizen.
[tr. Wodhull (1809)]

Whenever a wise man takes a good occasion for his speech, it is not a great task to speak well. You have a rapid tongue as though you were sensible, but there is no sense in your words. A man powerful in his boldness, one capable of speaking well, becomes a bad citizen in his lack of sense.
[tr. Buckley (1850)]

'Tis easy to be eloquent, for him
That's skilled in speech, and hath a stirring theme.
Thou hast the flowing tongue of a wise man,
But there's no wisdom in thy fluent words;
For the bold demagogue, powerful in speech,
Is but a dangerous citizen lacking sense.
[tr. Milman (1865)]

When wise men reason from sound principles,
They find it no hard task to reason well.
Thy tongue’s as fluent as the wisest man’s,
And yet thy argument is void of sense.
[tr. Rogers (1872), l. 253ff]

Whenso a man of wisdom finds a good topic for argument, it is no difficult matter to speak well; but thou, though possessing a glib tongue as if endowed with sense, art yet devoid thereof in all thou sayest. A headstrong man, if he have influence and a capacity for speaking, makes a bad citizen because he lacks sense.
[tr. Coleridge (1891)]

Whene'er a wise man finds a noble theme
For speech, 'tis easy to be eloquent.
Thou -- roundly runs thy tongue, as thou wert wise;
But in these words of thine sense is there none.
The rash man, armed with power and ready of speech,
Is a bad citizen, as void of sense.
[tr. Way (1898)]

Good words, my son, come easily, when he
That speaks is wise, and speaks but for the right.
Else come they never! Swift are thine, and bright
As though with thought, yet have no thought at all.
[tr. Murray (1902)]

Give a wise man an honest brief to plead
and his eloquence is no remarkable achievement.
But you are glib; your phrases come rolling out
smoothly on the tongue, as though your words were wise
instead of foolish. The man whose glibness flows
from his conceit of speech declares the thing he is:
a worthless and a stupid citizen.
[tr. Arrowsmith (1960)]

When a wise man chooses a sane basis
for his arguments, it is no great task to speak well;
but you have a glib tongue, as though in your right mind,
yet in your words there is no real sense.
The man who is influential by sheer aggressiveness, and knows how to speak,
proves to be a bad citizen -- for he lacks sanity.
[tr. Kirk (1970)]

When a clever man has a plausible theme to argue, to be eloquent is no great feat. But though you seem, by your glib tongue, to be intelligent, yet your words are foolish. Power and eloquence in a headstrong man can only lead to folly; and such a man is a danger to the state.
[tr. Vellacott (1973)]

Oh it's so easy for some to make speeches.
They pick a soft target and the words rush out.
Now listen you. Your tongue runs loose
Makes a plausible sound and might
Almost be taken for sense. But you have none.
Your glibness flows from sheer conceit.
Arrogant, over-confident and a gift -- yes --
A gift for phrases, and that makes you a great
Danger to your fellow men.
[tr. Soyinka (1973)]

A man who takes a fair basis for speaking,
a wise man, has no trouble speaking well;
you have a well-wheeled tongue, as though thinking,
but in the words you speak there is no thought.
A man empowered by daring and able to speak
becomes a bad citizen, devoid of reason.
[tr. Neuburg (1988)]

When some wise man has a fair cause
o present, to speak well is easy.
You have a tongue, glib like thought,
But no sense lies in your words.
The man that rashness prompts to speak
Proves an evil citizen and senseless.
[tr. Blessington (1993)]

Whenever a wise man sets out to argue an honest case
it's no great undertaking to argue well.
Your tongue runs smooth like a wheel, as if you were a man of reason,
but your words reveal no reason.
If he behaves recklessly, an able and articulate man
turns out to be a bad citizen because he lacks good sense.
[tr. Esposito (1998)]

When a prudent speaker takes up a noble cause, he’ll have no great trouble to speak well. You, on the other hand, have a tongue that runs on smoothly and sounds intelligent. But what it says is brainless. True, boldness can help a man speak powerfully, but he’ll turn out bad for the city because he'll have no sense.
[tr. Woodruff (1999)]

It's no great task to speak well, when a man's
Intelligent and starts well with good words.
But you: your tongue runs smoothly, as if you had
Some understanding. Yet your words are senseless.
A man like you, whose strength is that he's bold,
Who's good at speaking, too, can only make
a bad citizen -- for he lacks good sense.
[tr. Gibbons/Segal (2000)]

When a wise man has a good case to argue, eloquence is easy. As for you, though you think yourself clever and have a ready tongue, there is no intelligence in what you say. [A man whose power lies in brashness and who is a fluent speaker becomes a bad citizen if he lacks sense.]
[tr. Kovacs (2002)]

As for you -- your tongue is quick and your talk runs as if you had wit, but there is none in what you say. A man who confuses impudence with strength is a fool.
[tr. Rao/Wolf (2004)]

When a wise man is given the opportunity to speak, it’s no big problem to speak the truth. You, Pentheus, you are, of course an articulate man, or so you think, but your words lack logic. Audacity, strength and eloquence all on their own, make for a bad citizen -- a stupid one.
[tr. Theodoridis (2005)]

When a man who's wise in words starts his speech
from a proper course, it is no great task to speak well;
and you, spinning a tricky tongue, seem to make sense,
but there is no sense in what you are saying;
and a man who is bold, powerful and a clever speaker
makes for a bad citizen, if he has not the proper mind.
[tr. Valerie (2005)]

When a man of wisdom has good occasion to speak out and takes the opportunity, it's not that hard to give an excellent speech. You've got a quick tongue and seem intelligent, but your words don't make any sense at all. A fluent orator whose power comes from self-assurance and from nothing else makes a bad citizen, for he lacks sense.
[tr. Johnston (2008)]

When a wise man has an honest case to plead, then eloquence, I find, is very easy to achieve. You think yourself clever, and have a smooth tongue, but, your words are foolish. The man whose power lies in his conceit does not make a good citizen.
[tr. Robertson (2014)]

     It’s no great task for a wise man to speak well when the time comes, if he picks it carefully.      You hold yourself as if you’re one of these ready-tongued individuals. You’re not. Your words lack sense behind them.      Even the boldest speaker fails as a citizen when his words lack sense.
[tr. Pauly (2019)]

Wisdom from the wise surprises no one. But your clever tongue makes yuou seem wise when you have no understanding. Rash eloquence is society's disaster.
[tr. Behr/Foster (2019)]

Whenever a sophos man takes a good occasion for his speech, it is not a great task to speak well. You have a fluent tongue as though you are sensible, but there is no sense in your words. A bold and powerful man, one capable of speaking well, becomes a kakos citizen if he lacks sense.
[tr. Buckley/Sens/Nagy (2020)]