.NET MAUI Globalized String Markup Extension.

StringsExtension.cs

/// <summary>
/// Xaml extension providing globalized strings.
/// </summary>
/// <remarks>
/// Providing key itself if specified string is not found, 
/// </remarks>
public class StringsExtension : IMarkupExtension<string>
{
	/// <summary>
	/// Key of globalized strings.
	/// </summary>
	public string Key { get; set; }

	/// <summary>
	/// Culture in which strings is shown.
	/// </summary>
	/// <remarks>
	/// Default is <see cref="CultureInfo.CurrentUICulture" />.
	/// </remarks>
	[TypeConverter(typeof(CultureInfoConverter))]
	public CultureInfo Culture { get; set; } = CultureInfo.CurrentUICulture;

	private static readonly ResourceManager s_resourceManager = new(
		$"{nameof(ns)}.Resources.Strings", typeof(StringsExtension).Assembly); // Replace "ns" with your own project/namespace name

	public string ProvideValue(IServiceProvider serviceProvider) => s_resourceManager.GetString(Key, Culture) ?? Key;

	object IMarkupExtension.ProvideValue(IServiceProvider serviceProvider) => ProvideValue(serviceProvider);
}

See Resources in text files.
Assuming files are organized like

YourMauiProject\
    Platforms\
    Properties\
    Resources\
        AppIcon\
        ...
        Strings.restext
        Strings.ru.restext
        Strings.zh-Hans.restext
        ...
    App.config
    ...
    YourMauiProject.csproj

where resource text files are named "Strings" and stored in Resources folder just as line 24.
Make sure your resource text files ends with ".restext" and you add them to csproj:

<Project Sdk="Microsoft.NET.Sdk">
    ...
    <ItemGroup>
        <EmbeddedResource Include="Resources\*.restext" />
    </ItemGroup>
    ...
</Project>

See my example.