In today's eCommerce world, communicating with your customers is key. The way you interact with your customers has a HUGE impact on your brand. Remember that your customers are people too, and as cliché as it sounds-treat them the way you would want to be treated as a customer.
It's quite easy to forget this, and think people are asking stupid questions or wanting too much. But don't forget, you know your product/service inside and out, so it's easy to forget that at one point you might not have known everything there is to know either.
Communication is Key
If you can speak/write to customers in their own language that's great! Having a multi-lingual team is great to be able to help customers in more than one language. But don't worry, sometimes you can get by speaking only one language. If you do online support, you can always try Google Translate. It may not be perfect, but then maybe you can at least understand what they are having issues with.
Use positive language when possible. Avoid words like 'can't', 'don't' and 'won't' and try to phrase things in a more positive manner. Say you are sold out of an item. Rather than telling a customer "We can't get you that item" you might want to try "It looks like we'll be re-stocking in a week. I can place the order now, and then it'll be sent once it's stocked again!". This conveys the same message, but in a more positive manner.
Keep Your Cool
Remember, talk to customers with respect and treat them how you'd want to be treated. Even if a customer is angry or losing it, you need to keep a calm head. Stay respectful and continue to try to help them.
Don't forget your manners, even if people aren't responding with theirs!
Tone and Message is Key
You want to be sending brief messages in a timely manner. Wait too long and customers can get annoyed. They also don't want to be reading 1,000+ word responses if they don't need to. Try and address the customer by name if possible, as that's friendly and sometimes it helps make support seem less robotic. You can also mirror the way talk. If they're more professional than use a more professional tone. If they use GIFs and LOLs, then you can relax your tone a bit too.