Should I hold the same writing standards for international students as I do for domestic students?
There are two different types of value judgement that you are discussing here---communication goals and grading---and I think it's important to carefully separate them.


Communication Goals: I think that it is unfair to a person who isn't a native English speaker to not expect them to aim for the same goals of clear and lucid communication.  When giving feedback, don't brush over the language and grammar errors just because a person is not a native speaker.  They should have the same chance to learn and improve as anyone else.
Grading: Unless the class is about English grammar, making English quality a significant part of the grade is generally a bad way to evaluate a person's work.  If the language is bad enough that it interferes with your ability to understand the class-relevant content, then yes, that's a problem and should receive proportional demerits.  If the class-relevant content is clear, however, it's questionable to me whether any points should be taken off at all.


In short: grade very generously on language issues, but point out the mistakes so the student can continue to learn and improve.