app.xaml

<local:LocalizedStrings x:Key="S" />

AppResources.cs

public partial class AppResources
{
    private static readonly ResourceLoader _resourceLoader;       

        static AppResources()
        {
            _resourceLoader=new ResourceLoader();
        }

        internal AppResources()
        {
            
        }
}

AppResources.strings.cs

public string Game => _resourceLoader.GetString("Game");

csharp-usage.cs

var text = AppResources.Game;

localization-converter-1.cs

//directories representing the versions (e.g. 1.0, 1.0)
var dirs = Directory.GetDirectories(@"..\..\..\SecretProject-localization\export");

//the newest version
var version = dirs.Select(l => new Version(l.Split(Path.DirectorySeparatorChar).Last())).Max();

//all the localization keys
var keys = new HashSet<string>();

//each JSON file represents string for one language
var files = Directory.GetFiles(@"..\..\..\SecretProject-localization\export\"+ version).Where(l=>l.EndsWith(".json"));
foreach (var file in files)
{
    //RESW XML header
    var sb = new StringBuilder();
    sb.AppendLine(
        "<?xml version=\"1.0\" encoding=\"utf-8\"?><root>  <xsd:schema id=\"root\" xmlns=\"\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:msdata=\"urn:schemas-microsoft-com:xml-msdata\">    <xsd:import namespace=\"http://www.w3.org/XML/1998/namespace\" />    <xsd:element name=\"root\" msdata:IsDataSet=\"true\">      <xsd:complexType>        <xsd:choice maxOccurs=\"unbounded\">          <xsd:element name=\"metadata\">            <xsd:complexType>              <xsd:sequence>                <xsd:element name=\"value\" type=\"xsd:string\" minOccurs=\"0\" />              </xsd:sequence>              <xsd:attribute name=\"name\" use=\"required\" type=\"xsd:string\" />              <xsd:attribute name=\"type\" type=\"xsd:string\" />              <xsd:attribute name=\"mimetype\" type=\"xsd:string\" />              <xsd:attribute ref=\"xml:space\" />            </xsd:complexType>          </xsd:element>          <xsd:element name=\"assembly\">            <xsd:complexType>              <xsd:attribute name=\"alias\" type=\"xsd:string\" />              <xsd:attribute name=\"name\" type=\"xsd:string\" />            </xsd:complexType>          </xsd:element>          <xsd:element name=\"data\">            <xsd:complexType>              <xsd:sequence>                <xsd:element name=\"value\" type=\"xsd:string\" minOccurs=\"0\" msdata:Ordinal=\"1\" />                <xsd:element name=\"comment\" type=\"xsd:string\" minOccurs=\"0\" msdata:Ordinal=\"2\" />              </xsd:sequence>              <xsd:attribute name=\"name\" type=\"xsd:string\" use=\"required\" msdata:Ordinal=\"1\" />              <xsd:attribute name=\"type\" type=\"xsd:string\" msdata:Ordinal=\"3\" />              <xsd:attribute name=\"mimetype\" type=\"xsd:string\" msdata:Ordinal=\"4\" />              <xsd:attribute ref=\"xml:space\" />            </xsd:complexType>          </xsd:element>          <xsd:element name=\"resheader\">            <xsd:complexType>              <xsd:sequence>                <xsd:element name=\"value\" type=\"xsd:string\" minOccurs=\"0\" msdata:Ordinal=\"1\" />              </xsd:sequence>              <xsd:attribute name=\"name\" type=\"xsd:string\" use=\"required\" />            </xsd:complexType>          </xsd:element>        </xsd:choice>      </xsd:complexType>    </xsd:element>  </xsd:schema>  <resheader name=\"resmimetype\">    <value>text/microsoft-resx</value>  </resheader>  <resheader name=\"version\">    <value>2.0</value>  </resheader>  <resheader name=\"reader\">    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>  </resheader>  <resheader name=\"writer\">    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>  </resheader>");

    
    //read and parse the JSON file
    var content = File.ReadAllText(file);
    var j = JObject.Parse(content);
    
    IDictionary<string, JToken> strings = (JObject)j["strings"];

    //get all the localization keys
    foreach (var key in strings.Keys.Distinct())
    {
        var safeKey = key.Replace(".", "_DOT_").Replace("-", "_DASH_");

        //add an XML entry to the RESW file
        sb.AppendLine($"<data name=\"{safeKey}\" xml:space=\"preserve\">");
        sb.AppendLine($"<value>{strings[key].ToString().Replace("&","&amp;")}</value>");
        sb.AppendLine("</data>");

        //save the key for later use
        keys.Add(safeKey);
    }

    //end the RESW file
    sb.AppendLine("</root>");

    var pathsParts = file.Split(Path.DirectorySeparatorChar);
    var filenameParts = pathsParts.Last().Split('.');

    //write the RESW content to a correct file on the disk
    var suffix = filenameParts[0];
    var filename = @"..\..\..\SecretProject\strings\"+suffix+@"\Resources.resw";
    File.WriteAllText(filename, sb.ToString());
}

localization-converter-2.cs

var sbx = new StringBuilder();
sbx.AppendLine("namespace SecretProject.Localization");
sbx.AppendLine("{");
sbx.AppendLine(" public partial class AppResources");
sbx.AppendLine("    {");

foreach (var key in keys)
{
    sbx.AppendLine($"        public string {key} => _resourceLoader.GetString(\"{key}\");");
}

sbx.AppendLine("        public string GetString(string key) => _resourceLoader.GetString(key);");

sbx.AppendLine("    }");
sbx.AppendLine("}");

File.WriteAllText(@"..\..\..\SecretProject\Localization\AppResources.strings.cs", sbx.ToString());

LocalizedStrings.cs

public class LocalizedStrings
{
    public AppResources LocalizedResources { get; }

    public LocalizedStrings()
    {
        LocalizedResources = new AppResources();
    }
}

xaml-usage.xaml

<TextBlock Text="{Binding LocalizedResources.Game, Source={StaticResource S}}" />