Since my original answer was deleted as link-only and no-one cared to undelete although I improved it. Here it is again:

Found a workaround myself. See the github discussion https://github.com/orgs/wixtoolset/discussions/9081

PS: you need one file without language - a "neutral" file. So simply duplicate the english file or omit it:

Short steps (given your language file is named RtfTheme.wxl):

1. duplicate RtfTheme.wxl as RtfTheme.en-US.wxl

2. remove Culture="en-us" from RtfTheme.wxl

3. add <Payload Name="1033\thm.wxl" SourceFile="RtfTheme.en-US.wxl" />