Add source language metadata handling in merge command

Implemented logic to set the `source_language` field in resources during the merge process. The source language is determined from the first resource's metadata or defaults to "en" if not available. This enhancement ensures compatibility with xcstrings format and improves resource metadata management.

Author: WendellXY

langcodec-cli/src/merge.rs

@@ -66,6 +66,29 @@ pub fn run_merge_command(
     match converter::infer_format_from_path(output.clone()) {
         Some(format) => {
             println!("Converting resources to format: {:?}", format);
+            // Set source_language field in the resources to make sure xcstrings format would not throw an error
+            // First, try to get the source language from the first resource if it exists; otherwise, the first resource's language
+            // would be used as the source language. If the two checks fail, the default value "en" would be used.
+            let source_language = codec
+                .resources
+                .first()
+                .and_then(|r| r.metadata.custom.get("source_language").cloned())
+                .unwrap_or_else(|| {
+                    codec
+                        .resources
+                        .first()
+                        .map(|r| r.metadata.language.clone())
+                        .unwrap_or("en".to_string())
+                });
+
+            println!("Setting metadata.source_language to: {}", source_language);
+
+            codec.iter_mut().for_each(|r| {
+                r.metadata
+                    .custom
+                    .insert("source_language".to_string(), source_language.clone());
+            });
+
             if let Err(e) = converter::convert_resources_to_format(codec.resources, &output, format)
             {
                 println!("❌ Error converting resources to format");