A Chinese-language joke where near-identical sentences have different meanings: how do I translate this and find the humor?
I am a native Chinese speaker. Let me explain for you.

The humor: Every two sentences look the same but having totally different meanings.


  1、冬天：能穿多少穿多少；夏天：能穿多少穿多少。


Translate: Winter: Put on as much clothing as you can. Summer: Put on as fewer clothing as you can.

Explain: In the Winter sentence, 多少 is interpreted together, means "the quantity of the things that you have". In the sentence, it is exaggerated, and translate to "as much as you can". In the Summer sentence, 多少 should be interpreted separately as: 多 is an adjective "much", and 少 "a few", 多 is an adjective for 少, so 多少 means "very few".


  2、剩女产生的原因有两个，一是谁都看不上，二是谁都看不上


Background: 剩女 is a noun, means old girls (often older than 30) who are still single/unmarried. This is a slightly offensive word. 看上 means "falls in love with", 看不上 means "not falls in love with".

Translate: There are two reasons for being a 剩女: first, (she does) not fall for anybody; second, nobody falls for her.

Explain: The first sentence the "She" is an omission. For the second sentence, there is no omission, so 谁 is the body who is doing 看不上, and hence translates to "nobody falls for her".


  3、女孩给男朋友打电话：如果你到了，我还没到，你就等着吧；如果我到了，你还没到，你就等着吧。


Background: Most Chinese girlfriends are spoiled, and would demand much more from their boyfriends. And most Chinese take it for granted.

Translate: A girl gives her boyfriend a phone call: If you arrive, and I'm not there, you wait for me. If I arrive, and you're not there, you are going to wait for my punishment.

Explain: In Chinese, 等着 is a special word. In some occasions (depends on the context), 等着 translates to "give somebody a lesson". If someone is angry, he would say: 你给我等着. This means "I gonna get some helps and come back to punish you, you'd better stay here and do not go away." 


  4、单身的原因：原来是喜欢一个人，现在是喜欢一个人。


Yours is perfect.


  中国足球和中国乒乓球，一是谁都赢不了，二是谁都赢不了。


Your translation and understanding are both perfect.