Add localization documents

Author: Jack251970

localization-toolkit.md

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+Localization toolkit is for Flow C# plugin developers to improve their localization experience.
+
+## Initialization
+
+For C# Plugins, we need to install and reference [Flow.Launcher.Localization](www.nuget.org/packages/Flow.Launcher.Localization) by Nuget.
+
+## Build properties
+
+### `FLLUseDependencyInjection`
+
+Whether to use depenedency injection to get `IPublicAPI` instance. Default by false.
+
+If set to `false`, the `Main` class that implements **[IPlugin](/API-Reference/Flow.Launcher.Plugin/IPlugin.md)** or **[IAsyncPlugin](/API-Reference/Flow.Launcher.Plugin/IAsyncPlugin.md)** must have a [PluginInitContext](/API-Reference/Flow.Launcher.Plugin/PluginInitContext.md) property which must be at least `internal static`.
+
+If set to `true`, we can access `IPublicAPI` instance from `PublicApi.Instance` in the project by dependency injection.
+And the `Main` class does not need to have a [PluginInitContext](/API-Reference/Flow.Launcher.Plugin/PluginInitContext.md) property.
+(Not recommended for plugin projects because this will make plugins only compabible with Flow 1.20.0 or higher)
+
+## Usage
+
+### Main class
+
+`Main` class must implement [IPluginI18n](/API-Reference/Flow.Launcher.Plugin/IPluginI18n.md).
+
+If `FLLUseDependencyInjection` is set to `false`, `Main` class must have a [PluginInitContext](/API-Reference/Flow.Launcher.Plugin/PluginInitContext.md) property like:
+
+```
+public class Main : IPlugin, IPluginI18n // Must implement IPluginI18n
+{
+    internal static PluginInitContext Context { get; private set; } = null!; // At least internal static property
+    private static IPublicAPI API => Context.API;
+
+    ...
+}
+```
+
+### Localized strings
+
+With this toolkit, we can replace `Context.API.GetTranslation("flowlauncher_plugin_localization_demo_plugin_name")` with `Localize.flowlauncher_plugin_localization_demo_plugin_name()`.
+
+And we can also replace `string.Format(Context.API.GetTranslation("flowlauncher_plugin_localization_demo_plugin_used"), string.Empty, null, string.Empty)` with `Localize.flowlauncher_plugin_localization_demo_plugin_used(string.Empty, null, string.Empty)`.
+
+### Localized enums
+
+If you have enum types like `DemoEnum` that needs localization in displaying them on a combo box control. You can add `EnumLocalize` attribute to enable localiztion support.
+For all fields in this `EnumType`, if you want to specific one localization key for this field, you can use `EnumLocalizeKey` attribute; or if you want to specific one constant value for this field, you can use `EnumLocalizeValue` attribute.
+
+```
+[EnumLocalize] // Enable localization support
+public enum DemoEnum
+{
+    [EnumLocalizeKey("localize_key_1")] // Specific localization key
+    Value1,
+
+    [EnumLocalizeValue("localize_value_2")] // Specific localization value
+    Value2,
+
+    [EnumLocalizeKey("localize_key_3")] // If key and value both exist, will prefer localization key
+    [EnumLocalizeValue("localize_value_3")]
+    Value3,
+
+    [EnumLocalizeKey(nameof(Localize.flowlauncher_plugin_localization_demo_plugin_description))] // Use Localize class
+    Value4,
+}
+```
+
+Then you can get `DemoEnumData` class.
+
+In view model class which needs to display it on a combo box control, you can add two fields for binding `ItemSource` and `SelectedValue` like:
+
+```
+public List<DemoEnumData> AllDemoEnums { get; } = DemoEnumData.GetValues(); // ItemSource of ComboBox
+
+public DemoEnum SelectedDemoEnum { get; set; } // SelectedValue of ComboBox
+```
+
+```
+<ComboBox
+    DisplayMemberPath="Display"
+    ItemsSource="{Binding AllDemoEnums}"
+    SelectedValue="{Binding SelectedDemoEnum}"
+    SelectedValuePath="Value" />
+```
+
+If you want to update localization strings when culture info changes, you can call this function to update.
+
+```
+private void UpdateEnumDropdownLocalizations()
+{
+    DemoEnumData.UpdateLabels(AllDemoEnums);
+}
+```