Tesseract.js is the Javascript/Webassembly port of Tesseract. Tesseract.js intentionally inherits all code and default behaviors from the main Tesseract project, including the default language data. The language data used by default by Tesseract.js can be found here, and is copied directly from the main Tesseract project.

That being said, the language data used by Tesseract and Tesseract.js is simply a reasonable default. It is entirely possible to create and use custom language files that support any characters or language you like, or use language files created by other users. Instructions on how to use custom language files with Tesseract.js are here. For more information about language data, you should refer to the main Tesseract documentation, as the process of creating new language data is in the scope of the main Tesseract project rather than the JavaScript port.