See documentation to a KUndo2MagicString class.

Now KUndo2Command cannot accept a usual string as the operation text().
The supplied text must pass through the kundo2_i18n() function, which
tells the developer to use (qtundo-format) which is mandatory for all the
undo-related strings (because in some languages they should be shown in
different cases in undo/redo action and in undo history, and the
translators should use Magic Split for them).