Gtk+ provide several widget to let user input characters, such as GtkEntry, gtk multiline widget, and then you can using some method such gtk_entry_get_text() got what user input. But question is: what is the encoding of returned text on different locale?

The answer is that the encoding is utf8 no matter on which locale user input it.

#include <gtk/gtk.h>
#include <stdio.h>

void
cb_activate (GtkEntry *entry, gpointer user_data)
{
    const gchar *text;

    text = gtk_entry_get_text (GTK_ENTRY(entry));

    if (text != NULL)
        printf ("The text returned from entry: %s\n", text);
    else
        printf ("The text returned from entry: NULL\n");
}

int
main (int argc, char **argv)
{
    GtkWidget *window;
    GtkWidget *entry;

    gtk_init (&argc, &argv);

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title (GTK_WINDOW (window), "Test the encoding of text input by user");
    gtk_window_resize (GTK_WINDOW (window), 300, 100);
    g_signal_connect (window, "delete-event", G_CALLBACK (gtk_main_quit), NULL);
    g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);

    entry = gtk_entry_new ();
    g_signal_connect (entry, "activate", G_CALLBACK (cb_activate), NULL);
    gtk_container_add (GTK_CONTAINER (window), entry);

    gtk_widget_show_all (window);
    
    gtk_main ();

    return 0;
}