I've selected the fonts Liberation Sans for English (India) and Baloo Chettan 2 for Malayalam as default in Cell Styles. When I save the sheet in XLSX or XLS format and reopen it, fonts for both languages are getting changed to Arial! No issues while saving in ODS.
Steps to Reproduce:
1. Set Cell Style: Default fonts for English (India) and Malayalam to Liberation Sans and Baloo Chettan 2 [https://fonts.google.com/specimen/Baloo+Chettan+2] respectively.
2. Add some dummy English and Malayalam text [https://charapara.in/] in Sheet.
3. Save the sheet in XLSX or XLS and re-open it.
Fonts selected in Cell Styles: Default are getting changed to Arial.
Fonts selected in Cell Styles: Default should be preserved.
User Profile Reset: Yes
Version: 220.127.116.11 (x64)
Build ID: dd0751754f11728f69b42ee2af66670068624673
CPU threads: 4; OS: Windows 10.0; UI render: default; VCL: win;
Locale: en-IN (en_IN); UI-Language: en-GB
Created attachment 158690 [details]
default fonts selected in Cell Style
Created attachment 158691 [details]
fonts after re-opening the xlsx file
Created attachment 158692 [details]
ods sample file
Created attachment 158693 [details]
xlsx sample file
Wow, this is pretty basic stuff. There MUST be a default style in xlsx format isn't there? Yes - there is, but we don't export to it I guess.
<name val="Liberation Sans"/>