public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
        var style = new DataGridTextCellStyle
        {
            FontSize = 30,
            //FontFamily = "your font family here",
            FontAttributes = FontAttributes.Bold,
        };
        Dispatcher.Dispatch(() =>
        {
            var columns = this.datagrid.Columns;
            foreach (DataGridTypedColumn column in columns)
                if (column is DataGridTypedColumn)
                {
                    column.CellContentStyle = style;
                }
        });
    }
}

In order to display Japanese, Chinese characters, etc. you have to add and register the font in your MAUI application.  How to add and register fonts in your .NET MAUI application is described here: https://learn.microsoft.com/en-us/dotnet/maui/user-interface/fonts?view=net-maui-8.0 

Then you have to set the font family to the HeaderStyle, group style, etc.

 <telerik:DataGridNumericalColumn HeaderText="あああ"
                                  HeaderStyle="{StaticResource headerStyle}"
                                  PropertyName="Value"/>


    <ContentPage.Resources>
        <ResourceDictionary>
            <telerik:DataGridColumnHeaderStyle x:Key="headerStyle" TextFontFamily="JapaneseFont" />
        </ResourceDictionary>
    </ContentPage.Resources>