I made a new tab as shown in the tutorial:
$fields->addFieldToTab( 'Root.Content.Side', new HtmlEditorField( 'SideContent', 'Side Content' ) );
The tab has no proper name, it's displayed simply as "Side". When I set the locale to German, it should say "Neben-Inhalte".
I found a solution, but I wonder if there's a better one:
In FieldSet.php, I changed the addFieldToTab method, so that it no longer returns "void", but returns $tab, the tab that has been found or created by protected method FieldSet::findOrMakeTab().
Having the tab on hand, I can call $tab->setTitle( _t(...) ) :
$sideTab = $fields->addFieldToTab( 'Root.Content.Side', new TextareaField( 'FooterText', 'Footer Text', 3, 200 ) );
$sideTab->setTitle(_t('SiteTree.TABSIDE', "Side contents"));
Is there any better way to do this, without changing core functionality?