Thank you for your interest in Syncfusion products
Yes, our SfRichTextBoxAdv control supports multi-language input. For adding or changing a specific language keyboard layout kindly refer the below link
1. While entering the characters in specified language keyboard, the font family of the current selection in SfRichTextBoxAdv control should support the particular character code.
2. If the font-family does not support the particular character code then the entered character will be displayed as unreadable format.
3. In order to get those unreadable character as visible as actual character, select those characters and set the font family which supports those characters.
4. You can define font family which supports all the possible characters for the entire contents of SfRichTextBoxAdv control, to avoid these type of character visibility failures.
Please let us know if you have any other question,