成语 versus 俗语 versus 谚语, what is the difference?
成語 and 諺語 are well established common expressions , well known by the whole population. 俗語 is more often, limited to regional use.

成語 mostly refer to Chinese idiom.
諺語 can be from different countries.
Chinese 成語 are uniformly four characters phrases.
For 俗語 and 諺語,  number of character are flexible, some come in two parts.

Examples:

俗語 : &quot;有便宜唔好使頸&quot; (Cantonese) : &quot;If there's benefit to be have, don't be stubborn.&quot;
諺語: &quot;神要毀滅某人，必先使其瘋狂&quot; (western): &quot; &quot;Those whom the gods wish to destroy they first make mad.&quot;


諺語 (proverb) include  &quot;common sense&quot; and &quot;words of wisdom&quot;. from known or unknown origins.
俗語 (folk saying) include &quot;situation description&quot;, &quot;common sense&quot; and &quot;words of wisdom&quot;.
成語 (idiom) include  &quot;situation description&quot; , &quot;common sense&quot;, and &quot;words of wisdom&quot;.  Mostly came from known historical references, like poems, classical literature or famous quotes by historic figures .

Edit:
熟语 (Well known phrases) include:
惯用语(common phrase), usually contain three characters, for example: &quot;打秋風&quot;，&quot;拍馬屁&quot; , 成語, 諺語, 俗語, 歇后语(two-part allegorical saying), for example: &quot;和尚打傘，無法無天&quot; and 格言(motto), usually from famous people, for example: &quot;We have nothing to fear but fear itself.&quot; by Franklin D. Roosevelt.