<UserControl x:Class="MyControls.LanguageSelectView" ... >
...
<ComboBox
        ItemsSource="{Binding ViewModel.AvailableCultures, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:LanguageSelectView}}}"
        SelectedItem="{Binding SelectedCulture, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:LanguageSelectView}}}"
        >
    <ComboBox.ItemTemplate>
    ...
    </ComboBox.ItemTemplate>
</ComboBox>
</UserControl>

public static readonly DependencyProperty SelectedCultureProperty = DependencyProperty.Register(nameof(SelectedCulture), typeof(CultureInfo),
        typeof(LanguageSelectView), new FrameworkPropertyMetadata(null) {BindsTwoWayByDefault = true, DefaultUpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged});

public CultureInfo SelectedCulture
{
    get { return (CultureInfo) GetValue(SelectedCultureProperty); }
    set { SetValue(SelectedCultureProperty, value); }
}

[Import]
public LanguageSelectViewModel ViewModel { get; set; }

...
<DataGrid ItemsSource="{Binding Users}" AutoGenerateColumns="False" Name="DataGrid" SelectionChanged="DataGrid_OnSelectionChanged">
    <DataGrid.Columns>
        <DataGridTemplateColumn Header="{wpf:Localize PrefferedLanguage}">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <localization:LanguageSelectView SelectedCulture="{Binding SelectedCulture}" />
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>
</DataGrid>
...

public class UserViewModel
{
    ...

    public CultureInfo SelectedCulture
    {
        get { <Getter> }
        set { <Setter> <-- It's not getting called! }
    }
}