Read the user's preferred language from the OS (#382)

app/MindWork AI Studio/Tools/PluginSystem/PluginLanguage.cs

@@ -6,12 +6,16 @@ public sealed class PluginLanguage : PluginBase, ILanguagePlugin
 {
     private readonly Dictionary<string, string> content = [];
     private readonly List<ILanguagePlugin> otherLanguagePlugins = [];
+    private readonly string langCultureTag;
     
     private ILanguagePlugin? baseLanguage;
     
     public PluginLanguage(bool isInternal, LuaState state, PluginType type) : base(isInternal, state, type)
     {
-        if (this.TryInitUITextContent(out var issue, out var readContent))
+        if(!this.TryInitIETFTag(out var issue, out this.langCultureTag))
+            this.pluginIssues.Add(issue);
+        
+        if (this.TryInitUITextContent(out issue, out var readContent))
             this.content = readContent;
         else
             this.pluginIssues.Add(issue);
@@ -65,4 +69,62 @@ public sealed class PluginLanguage : PluginBase, ILanguagePlugin
         value = string.Empty;
         return false;
     }
+    
+    /// <summary>
+    /// Tries to initialize the IETF tag.
+    /// </summary>
+    /// <param name="message">The error message, when the IETF tag could not be read.</param>
+    /// <param name="readLangCultureTag">The read IETF tag.</param>
+    /// <returns>True, when the IETF tag could be read, false otherwise.</returns>
+    private bool TryInitIETFTag(out string message, out string readLangCultureTag)
+    {
+        if (!this.state.Environment["IETF_TAG"].TryRead(out readLangCultureTag))
+        {
+            message = "The field IETF_TAG does not exist or is not a valid string.";
+            readLangCultureTag = string.Empty;
+            return false;
+        }
+        
+        if (string.IsNullOrWhiteSpace(readLangCultureTag))
+        {
+            message = "The field IETF_TAG is empty. Use a valid IETF tag like 'en-US'. The first part is the language, the second part is the country code.";
+            readLangCultureTag = string.Empty;
+            return false;
+        }
+        
+        if (readLangCultureTag.Length != 5)
+        {
+            message = "The field IETF_TAG is not a valid IETF tag. Use a valid IETF tag like 'en-US'. The first part is the language, the second part is the country code.";
+            readLangCultureTag = string.Empty;
+            return false;
+        }
+        
+        if (readLangCultureTag[2] != '-')
+        {
+            message = "The field IETF_TAG is not a valid IETF tag. Use a valid IETF tag like 'en-US'. The first part is the language, the second part is the country code.";
+            readLangCultureTag = string.Empty;
+            return false;
+        }
+        
+        // Check the first part consists of only lower case letters:
+        for (var i = 0; i < 2; i++)
+            if (!char.IsLower(readLangCultureTag[i]))
+            {
+                message = "The field IETF_TAG is not a valid IETF tag. Use a valid IETF tag like 'en-US'. The first part is the language, the second part is the country code.";
+                readLangCultureTag = string.Empty;
+                return false;
+            }
+        
+        // Check the second part consists of only upper case letters:
+        for (var i = 3; i < 5; i++)
+            if (!char.IsUpper(readLangCultureTag[i]))
+            {
+                message = "The field IETF_TAG is not a valid IETF tag. Use a valid IETF tag like 'en-US'. The first part is the language, the second part is the country code.";
+                readLangCultureTag = string.Empty;
+                return false;
+            }
+        
+        message = string.Empty;
+        return true;
+    }
 }