On Windows 10 Mobile, this issue is most likely to involve East Asian languages (Chinese, Japanese, Korean). This issue typically involves text in Middle East or Asian languages (Arabic, Chinese, Hindi, etc.). When running certain apps on Windows 10 desktop or Windows 10 Mobile, some characters display as a square or rectangular box, or as a box with a dot, question mark or “x” inside, while the same app running on earlier Windows or Windows Phone versions did not have this problem.
