MainPage.xaml

<Page
    x:Class="OverridingAppLanguage.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:OverrideLanguage"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <TextBlock x:Uid="Hello" HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="60">XXXX</TextBlock>
        
        <StackPanel>
            <Button Click="SetFrench">Set French Language</Button>
            <Button Click="SetEnglish">Set English Language</Button>
            <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="30" x:Name="Index">init</TextBlock>
        </StackPanel>
    </Grid>
</Page>