If you have bought a theme you should have support for other languages. See the chapter Internationalization in the theme developer handbook for more information, or e.g. more specific on translation functions like __() the Code Reference.
If you independently developed a theme, you probably forgot about this element. If not, check if your theme not have strings in theme without __(), if it is true, you should add all strings to functions like this – see above linked resource for an overview.
Furthermore, the Theme Handbook has an informative chapter on how to localize, with information about logic behind it and tools to use, you might find interesting.