Created attachment 171683 [details]
Screenshot with LibreOffice Dark color scheme
Via Tools > Options > LibreOffice > Application Colors: AutoSpellcheck (under Text Document) it is possible to change the appearance of red-lines (wavy lines that indicate spelling errors). Changing the color has no effect until the paragraph is checked again (eg after changes to the text or return at the end).
In the screenshot the Arabic text was checked with Dark Red font color. After changing it to yellow the wrong language under Hidden Text was added.
I confirm it with
Version: 184.108.40.206.alpha0+ (x64) / LibreOffice Community
Build ID: 1675a68526c43c6c6e4dc850ee911f0c1de75c88
CPU threads: 4; OS: Windows 10.0 Build 19042; UI render: Skia/Raster; VCL: win
Locale: de-DE (de_DE); UI: en-GB