Mirror indicator turns on other side

If it's the turn signals inside the mirror, you usually have to replace the mirror as an assembly due to how they are implemented in the mirror assembly. We can't reference the part you need without knowing your cars model year. That said if you search online for it, you'll find some. You'll also need to paint the replacement part, so you will need a paint like PerfectMatch and an automotive grade clearcoat to finish the job. If you opt for a used mirror and not a new one (which will have a base coat ready for paint), add grey primer to the mix.

Just remember that if the car has features like BSD that rely on the mirror, you'll need to get a bidirectional scanner with ADAS support to code the new mirror to the car. I've run into this on a Kia Optima with the BSD feature after the issue remained, and my scan tool (non-ADAS capable) was able to pick up a C DTC, meaning the mirror needed to be calibrated to the car. You could paint the mirror and ask a shop to calibrate it to the car after installation if you do not want to front the cost of the ADAS scan tool.

@fehmi1912 for that there is a setting in your Golf firmware. To change it, you would need a VW scanner. This is one of those case where I think the local dealer is actually of more help. They can reset/set the indicator to the proper side by changing the values of the Vehicle electrical system control. Now interestingly enough, I wonder how it changed on your Golf.