Hi Mattia,

Thank you for reporting this and for the detailed screenshots, they helped a lot in pinpointing the issue.

We confirmed this is a bug in the Gantt control: the Localization-AdvancedAssignResources property is not wired up correctly internally, so the value you set (via the .resx file or declaratively) never reaches the client-side "Assign" button, which always falls back to its default English text "Assign". The fix has been implemented and is scheduled to be released in R3 2026 SP1.

In the meantime, you can override the button's text on the client side. For example, using the Gantt's taskEditing client-side event, you can locate the "Assign" link element in the edit form and replace its text with your localized string right before/when the dialog is shown:

<script>
    function onTaskEditing(sender, args) {
        var container = args.container || (args.get_container && args.get_container());
        if (!container) {
            return;
        }

        $(container)
            .find('[data-container-for="resources"] a.radButton')
            .text("Assegna");
    }
</script>
<telerik:RadGantt RenderMode="Lightweight" runat="server" ID="RadGantt1"
    Culture="it-IT"
    OnClientTaskEditing="onTaskEditing"
    ...
>
...

Please let us know if you need help implementing the workaround, or if you'd prefer to wait for the official fix in R3 2026 SP1.
Thank you for your patience!

Also, as a small token of gratitude for reporting this, I rewarded you with Telerik points.

Regards,
Rumen
Progress Telerik