This is Microsoft's explanation:
Some key improvements have been made in Windows 10 that affect fonts; more details on this are provided below. A side effect of these changes is that a small subset of existing apps created for earlier versions of Windows or Windows Phone may be affected.
Some fonts that were included in Windows Phone 8.1 are not included in Windows 10 Mobile.
Some fonts that previously were included in every Windows desktop system have been moved into optional font packages, and so may not be present on all Windows 10 desktop systems.
More details on these changes are provided below. In certain apps, these changes can lead to the “square box” symptoms when displaying certain international text.
All Windows 10 editions include fonts that provide broad language support, and the Windows platform includes font fallback mechanisms designed to ensure that text in any language always displays with legible glyphs rather than boxes. But some apps may take direct dependencies on particular fonts for displaying certain Unicode characters and do not utilize the font fallback mechanisms provided by Windows. In some cases, these apps have taken direct dependencies on fonts that are not present by default on all Windows 10 systems. Because the font that the app is trying to use is not present on the system, some other font gets used to display the text instead, and that font may not support all of the characters being displayed. When a character is displayed using a font that doesn’t support that character, a default “not defined” glyph from that font is used. The “not defined” glyph in most fonts has the appearance of a rectangular box, or some variation of that.
Source: https://support.microsoft.com/en-us/help/308380...
You can try the following:
Open Control Panel
Open Open "Clock, Language, and Region".
Open Open "Change keyboards or other input methods" of "Region and Language".
Open "Change system locale..." of the "Administrative" tab.
If a dialog box (User Account Control) appears to ask your permission to continue,
Click "Continue"
Select the language of your choice from the drop-down menu.
Your default language will not change by selecting the language here.
Restart Windows.