To quote for 4.27 and 5.1:
Text Formatting and Fonts in UMG for Unreal Engine | Unreal Engine 5.1 Documentation
By default, UMG text widgets use a font that ships with Unreal Engine. This gives you something to work with out of the box so you can get started quickly. However, this built-in font has some limitations; for example, it only supports a small set of languages. For most projects, especially those that require UIs with non-English text, you will want to import your own custom fonts into Assets and set up your text widgets to use your fonts.
Does this mean the default Roboto font excludes people who don’t read english? When I develop a plugin containing UI functionality to distribute worldwide, I don’t want to bloat it with designs and fonts, or be forced to develop a font to provide proper UX to customers because this should be in the engine by default.
Request: Add multi language support out of the box.