A regression in September 2020 CU for for SharePoint 2019 was identified.
Users who install SharePoint in non-english language or install non-english language packs can’t work with some modern pages in non-english sites.
To work around this issue, international users can switch back to classic mode.
KB 4484505 has been updated include the information in the “Known Issues” section.
Microsoft is aware of the problem and is working on a fix.