I want to know how I could interrupt them and explain that there was a miscommunication without being rude.
Part of the offense that can be taken by the other party when you correct them is whether you imply that they are at fault for the misunderstanding. This is perfectly embodied by your title:
How can I tactfully point out that someone misunderstood me?
Someone misunderstood you. Someone did something wrong. That implies that the issue lies with this person.
Contrast this to:
How can I tactfully point out that my message did not get across?
The message did not get across. Maybe it was received wrongly. Maybe it was sent wrongly. You leave it unspoken as to who is to blame for the misunderstanding, you're simply pointing out that there is a misunderstanding regardless of who made the mistake.
Which you can further contrast to:
How can I tactfully point out that I have misexplained something?
Here, you are explicitly blaming yourself for the miscommunication. This is the most applicable to your case, since you explicitly describe that you formulated your question badly.
Similarly, you can use "that I may have misexplained something" if you're not quite sure if you misexplained but you suspect you did.
Because this is an IM chat, you need to respond to it differently than you do in real life.
The main difference with IM is that your message is unseen until you've actually finished (and thus sent) it. This works against you, because while you are still typing out your correction, the other person is still unaware that you've realized there has been a miscommunication. The other person may be in the process of typing a lengthy response, unaware that they are writing a response that isn't actually necessary.
Because of this, I suggest immediately interjecting. Something along the lines of:
Wait, I think I misexplained.
Doing so in a spoken conversation may come across as rude because it interrupts the other person. But that rules doesn't quite apply in an IM chat. You are interjecting as a kindness to the other person, to prevent them writing a long answer that you didn't actually need.
This prompts the other person to stop what they're doing and wait for your next message, for which you can then take the time to write an accurate (and not rushed) correction:
What I meant to ask was how I was supposed to translate the English text that I had in the requirements into Spanish text for the website, since I don't speak Spanish
If you want, you can then add an explanation for why you're correcting your statement:
On rereading my initial question, I realized it could be interpreted as me asking how to pick the site language.
Note that I used a fairly formal phrasing because I'm not sure of your region's company culture. In our Slack chat, things are considerably more casual than this. I would casually write something along the lines of:
- I meant [correct version], not [misinterpretation]. My bad.