I accidentally discovered the following “feature”: All LCID Titles in SubArea must have matching LCID in Group Titles.
Add a group in sitemap, like this:
<Group Id="Demo">
<Titles>
<Title LCID="1033" Title="Demo"/>
</Titles>
<SubArea Id="DemoPage" Icon="iconUrl" Url="someUrl">
<Titles>
<Title LCID="1033" Title="Demo Page"/>
<Title LCID="1060" Title="Demo Stran"/>
</Titles>
</SubArea>
</Group>
The sitemap will upload normally and pass all validation, but after switching to language 1060, the error will be shown. The cause is missing LCID 1060 in Group Titles.
The correct XML for this example is:
<Group Id="Demo">
<Titles>
<Title LCID="1033" Title="Demo"/>
<Title LCID="1060" Title="Demo"/>
</Titles>
<SubArea Id="DemoPage" Icon="iconUrl" Url="someUrl">
<Titles>
<Title LCID="1033" Title="Demo Page"/>
<Title LCID="1060" Title="Demo Stran"/>
</Titles>
</SubArea>
</Group>
