One is the purpose you mentioned, marking a project progress as “Complete” → the translations would be “erledigt”
The other is not shown in the usual UI, but shown to accessibility users for the check mark button on tasks, telling them the button means “marking a task as complete” → the translations would be “erledigen”
The mobile team is going to disambiguate them so as we can use different translations.
By now, we have applied the suggestion replacing “erledigen” with “erledigt” since the first case is the most visible one. The new translation will go live on the next deploy!