How should I offer corrections on English for coworkers who are not native speakers?

When is it appropriate to ask them to correct their spelling and grammar? How would you suggest I do this in a way that is respectful of their current English skills?

Ask them first. I have worked with many non-native English speakers from around the world. The following are normally true, from India to Europe to Mexico:

• Most are somewhat self conscious about their English
• Most are very interested in improving their English
• Nearly all will not actively solicit feedback
• Nearly all will be interested in help/guidance if offered appropriately

How do you get there? It's really pretty simple.

1. Be respectful overall. It takes a lot of vulnerability for someone to basically say, "I don't know your language very well but am wiling to receive help." Being universally respectful goes a long way towards making someone feel ok with this.
2. Ask in a non-threatening way. Make sure that when you ask, you give the ultimate choice to the non-native speaker. This is especially important when talking to people from less direct cultures such as India.
   • "Is it ok if I correct your English?" <-- bad, you imply the answer
   • "This is wrong." <-- bad, very confrontational and likely to humiliate/embarrass
   • "Do you want me to point out any stylistic differences I'd make in English?" <-- good, this is more open ended
   • "Can I change this comment slightly? It's not 100% clear to me." "Sure" "By the way, do you want me to be more proactive on this sort of thing?" <-- Good, lets them decide
3. Respect their wishes. Unless it's a business issue be respectful of what the answer is. Keep in mind everything customer/external facing is a business issue.
4. Don't overwhelm anyone. Sometimes people are... not as good at English. You could find yourself continually "fix this" if pointing out mistakes during code reviews/etc. Make sure you have the appropriate level and quantity of instruction.
5. Show interest in their culture/language. This is optional but if you are actively trying to learn about their culture, language, etc, you are going to have a lot more success in helping them out.

Over the past years multiple groups and teams I've worked with internationally have solicited my help with English in some form. Often this comes in the form of proof reading documents.