It sounds like you are saying that you have trouble understanding directions from this senior person, and you are concerned that this lack of understanding could make you look bad. You're likely correct.
- It isn't your job to correct his English,
continuing to do that may be bad for your career.
- Doing the wrong work (or the work wrong) is also bad for your career.
Your goal is understanding the instructions.
You should repeat back what you hear from him to make sure the directions are clear in both of your minds. This is not the same as asking him to clarify (though they are similar).
If you think that he misspoke when he said, "Do X but be careful of Y." You can repeat it back in the corrected form as "You want me to do Z and be careful of Y. Did I understand you?"
Bob Jarvis has a good example; it is in a comment to your question.
How you do this is important. In other words, where is your 'heart' in the exchange?
Did you noticed that I didn't mention his flaws in my opening paragraph?
Your sole purpose (only desired outcome) in this exchange is to make sure you understand his instructions. You don't do this to point out his weak English, and you don't do it to point out his mistake. Let me be clear: this not an opportunity to make yourself look good in any way.
As you work together, you will grow to understand how he communicates and this will make things easier for you.
Depending on how this goes, it may be good for you to email him your understanding. Many ESL people are better with written than spoken English.
Whatever part of this is his problem, if any, can be addressed by people more senior than him. This advice is for you - because you asked.