Here are some ways you could describe it in English:
* Silver anklet: This is a general description that will be understood.
* Traditional South Indian anklet: This is more specific and mentions the cultural origin.
* Thoppul (Tamil for anklet): You could simply mention the word "thoppul" and add the meaning in parentheses for clarity.
Ultimately, the best way to describe "thoppul" in English depends on the context and your audience.