Fix docs

.github/workflows/rubocop.yml

@@ -10,7 +10,7 @@ permissions:
   contents: read
 
 jobs:
-  test:
+  rubocop:
     name: RuboCop
     runs-on: ubuntu-latest
 

.github/workflows/yard.yml

@@ -0,0 +1,28 @@
+name: YARD
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+permissions:
+  contents: read
+
+jobs:
+  yard:
+    name: YARD
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: '2.6.10'
+        bundler-cache: false
+        rubygems: latest
+    - name: Lint YARD documentation
+      run: |
+        gem install yard
+        yard doc --fail-on-warning --no-output --no-progress

lib/utils.rb

@@ -21,7 +21,7 @@ def self.symbolize_keys(obj)
     # Converts a language code to its corresponding ISO 639-2 (alpha-3) code.
     #
     # @param [String] language The ISO 639-1 (alpha-2) or ISO 639-2 (alpha-3) language code.
-    # @return [String] The ISO 639-2 (alpha-3) language code, or `nil` if the code is not found.
+    # @return [String, nil] The ISO 639-2 (alpha-3) language code, or `nil` if the language is not found.
     def self.language_alpha3(language)
       ISO_639.find(language)&.alpha3
     end