[Autofill][Translate] Added documentation of source language code.

Bug: 1142413, 1150277
Change-Id: I19a5f90933e5da7e9d2ea79e7c1fff7c48002cd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550383Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/master@{#829867}

components/autofill/core/common/language_code.h

@@ -9,14 +9,15 @@
 #include <string>
 #include <utility>
 
-#include "base/logging.h"
 #include "base/ranges/algorithm.h"
 #include "base/types/strong_alias.h"
 
 namespace autofill {
 
-// A LanguageCode is a two-letter lowercase abbreviation according to ISO 639-1
-// or "und", which is the ISO 639-2 code for "undetermined".
+// Following the implicit conventions in //components/translate, a LanguageCode
+// in  is a lowercase alphabetic string of length up to 3, or "zh-CN", or
+// "zh-TW". A non-exhaustive list of common values is
+// translate::kDefaultSupportedLanguages.
 class LanguageCode
     : public base::StrongAlias<class LanguageCodeTag, std::string> {
  private:
@@ -33,10 +34,8 @@ class LanguageCode
 
  private:
   void Check() {
-    LOG_IF(ERROR,
-           !(empty() ||
-             (base::ranges::all_of(value(), &islower) && length() == 2) ||
-             value() == "und" || value() == "zh-CN" || value() == "zh-TW"))
+    DCHECK((length() <= 3 && base::ranges::all_of(value(), &islower)) ||
+           value() == "zh-CN" || value() == "zh-TW")
         << "Unexpected language code '" << value() << "'";
   }
 };

components/translate/core/browser/language_state.h

@@ -52,11 +52,15 @@ class LanguageState {
   // Returns true if the current page in the associated tab has been translated.
   bool IsPageTranslated() const { return original_lang_ != current_lang_; }
 
-  void SetOriginalLanguage(const std::string& language);
+  // Returns the original language represented as a lowercase alphabetic string
+  // of length 0 to 3 or "zh-CN" or "zh-TW".
   const std::string& original_language() const { return original_lang_; }
+  void SetOriginalLanguage(const std::string& language);
 
-  void SetCurrentLanguage(const std::string& language);
+  // Returns the current language represented as a lowercase alphabetic string
+  // of length 0 to 3 or "zh-CN" or "zh-TW".
   const std::string& current_language() const { return current_lang_; }
+  void SetCurrentLanguage(const std::string& language);
 
   bool page_needs_translation() const { return page_needs_translation_; }