One of the first things I learnt to say, when I lived in China, was "I can't speak Chinese". In fact, I learnt to say it so well, that the person I was speaking to wouldn't believe me!
Motivation
Why is it important to learn how to say that you can't speak Chinese?
If you're a beginner and want to try out your Chinese skills, people will often assume you can speak and will start rattling off Chinese at you. Unless your Chinese is pretty good, the chances are that most of what they say will go right over your head. You need to be able let them know that you can't really speak Chinese fluently.
Lesson
First, here is how to say "I don't understand" (literal meaning in brackets):
Wǒ tīng bù dǒng
(I hear not understand)
Next, here is how to say that you can't speak Chinese. Use this if you really can't speak at all:
Wǒ bú huì shuō pǔtōnghuà
(I not can speak Chinese)
If you know a little bit of Chinese, you might say:
Wǒ huì shuō pǔtōnghuà yìdiǎn
(I can speak Chinese a little)
An alternative, which I sometimes use, is:
Wǒ pǔtōnghuà shuō de bù hǎo
(I/My Chinese speak no good)
My favourite is actually:
Wǒ huì shuō yìdiǎn, kěshi wǒ tīng bù hǎo
(I can speak a little, but I hear no good)
I use this to explain that while I can talk to them a little bit, I can't actually understand them when they start talking back to me at a rate of knots. If you can understand a little bit and they are just talking too fast, try adding:
Nǐ shuō tài kuài le. Màn màn shuō
(You talk too quickly. Slowly slowly speak).
I'm sure that's not the politest way of saying it, but foreigners can get away with it - I've never had a problem using it!
Lesson Notes
yìdiǎn is often prounounced as yìdiǎr, especially in Beijing.
I've used pǔtōnghuà for Chinese, which in fact means "common language". This is the most commonly used term for Chinese in mainland China. You can swap this for hànyǔ, which means "Chinese language", especially if you are dealing with people that aren't from the mainland.
You shouldn't use zhōngwén as this is the Chinese written language. That's a trap I used to fall into.
Interestingly, the actual translation for Mandarin appears to be either Guānhuà (speech of officials) or Běifānghuà (northern dialect), but I've never heard these used. Stick to pǔtōnghuà or hànyǔ.
You don't have to actually say Chinese. All of the above work even if you drop the pǔtōnghuà from the phrases. The average Chinese person probably wouldn't say it. For example, they'd probably say:
Wǒ huì shuō yìdiǎn
(I can talk a little)
rather than the full:
Wǒ huì shuō pǔtōnghuà yìdiǎn
Also, I use shuō for talk, which in my experience is the most commonly used term. However, at times jiǎng is used instead of shuō.
Keywords: shuo, jiang, wo hui shuo, wo bu hui shuo, putonghua, hanyu, zhongwen, guanhua, beifanghua, yidian, ting bu dong, shuo de bu hao, ting bu hao, shou tai kuaile, man man shuo
Final Thoughts
There are quite a few variables around this, but learning the different ways to say that you can't speak, or can only speak a little, is one of the things that it's worth learning early on.