HR Wants Me to Avoid My Language

I (34M) work in an office in a technical field with approximately 30 coworkers and higher ups.

The problem is I have a coworker (28F) Sarah. The problem started before Christmas

when I went into the breakroom to answer a call from family overseas. We talked in my

native language for about 10 minutes, about family and friends from back home,

and we were wrapping up the call. When I hung up, I saw Sarah looking at me with an angry

look on her face. I asked her what's wrong and she accused me of being a racist. I was very confused,

and she then said I said the 'N word', which confused me even more. She ran off and

in a few minutes the HR director walked to my desk and took me to her office.