with some settings the TextRenderComponent is unable to render some glyphs, I created a sample project where the issue can be observed, it can be reproduced with 4.7,4.8,4.9 version of the engine.
Select TextRenderActor2 from the World Outliner and observe :“AND YOUR MOUSE TO LOOK AROUND
И ОСМАТРИВАЙСЯ ВОКРУГ С ПОМОЩЬЮ МЫШИ” text on the main viewport
Go to the content browser, find and open Font’/Game/StarterContent/Arial.Arial’
Change UnicodeRange from the defaul Cyryllic charset(0400-04ff) to → 0200-04FF
Click Asset and Reimport Arial, you can observe that engine created 2 pages with glyps and most of russian glyps are at the second page
Go to the main viewport and observe that the text is broken
I was able to reproduce the issue of the Russian lettering occurring inside of my text editor window as well as viewport.
However, what I suspect is that this is intended behavior.
As per step 4, where you change your UnicodeRange, you have changed your range for your text to be looked at. For instance, if you had one sheet of paper with all of the letters of the alphabet and you told the editor I want you to look at that page for input. The text editor would look at that one page for it’s input and find whatever letter you wanted to type. By changing the range to a much larger area, like multiple pages, where there are many different symbols, the editor then tries to approximate where to look. Since the editor does not essentially recognize or understand what letters are it looks for symbols and shapes that look close. Since you have given it more input it now tries to figure out what letter you were intending to use and draws it.
The recommended workflow for this is to use only one sheet. That way the margin for machine error is limited. From there you will need to tweak settings for height and kerning to type what you are expecting in the viewport.