feat: add i18n support with zh-Hant and zh-CN translations

[Jacky97s]

Summary

• Add I18n utility class with UTF-8 ResourceBundle loading and locale parsing
• Extract all user-facing strings from renderers (Console, Markdown, HTML, AsciiDoc) into .properties resource bundles
• Add --lang CLI option to select output language (en, zh-Hant, zh-CN)
• Add CJK-aware display width calculation for correct console alignment with Chinese characters
• Add Traditional Chinese (zh-TW) and Simplified Chinese (zh-CN) translation files
• Set HTML lang attribute dynamically based on locale

Usage

# Default (English)
openapi-diff old.yaml new.yaml

# Traditional Chinese
openapi-diff old.yaml new.yaml --lang zh-Hant

# Simplified Chinese
openapi-diff old.yaml new.yaml --lang zh-CN

Test plan

• All 266 existing tests pass with default English locale
• Verify console output alignment with Chinese characters
• Verify HTML output includes correct lang attribute for each locale
• Verify --lang CLI option selects correct translations

---

Summary by cubic

Add internationalization to all renderers and the CLI, with English, Traditional Chinese (zh-Hant), and Simplified Chinese (zh-CN) output. Adds a --lang option, fixes console centering for CJK, and sets the HTML lang attribute per locale.

• New Features
  • Introduced I18n utility with UTF-8 ResourceBundle loading and locale parsing.
  • Replaced hardcoded strings in Console/Markdown/HTML/AsciiDoc with bundle messages.
  • Added translation files: messages.properties (en), messages_zh_TW.properties (zh-Hant), messages_zh_CN.properties (zh-CN).
  • Added --lang CLI option to select output language (en, zh-Hant, zh-CN), defaulting to en.
  • Implemented CJK-aware display width for proper console alignment with Chinese characters.
  • HTML output now sets the lang attribute based on the selected locale.

^{Written for commit b08e836. Summary will update on new commits.}

[cubic-dev-ai]

2 issues found across 10 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="core/src/main/java/org/openapitools/openapidiff/core/output/ConsoleRender.java">

<violation number="1" location="core/src/main/java/org/openapitools/openapidiff/core/output/ConsoleRender.java:362">
P2: New CJK centering helper is not null-safe and can throw NPE during rendering/formatting.</violation>
</file>

<file name="core/src/main/java/org/openapitools/openapidiff/core/output/I18n.java">

<violation number="1" location="core/src/main/java/org/openapitools/openapidiff/core/output/I18n.java:20">
P2: `I18n` uses unsynchronized mutable static locale/bundle state, which is not thread-safe and can leak or mismatch language state across concurrent operations.</violation>
</file>

---

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[cubic-dev-ai]

P2: New CJK centering helper is not null-safe and can throw NPE during rendering/formatting.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At core/src/main/java/org/openapitools/openapidiff/core/output/ConsoleRender.java, line 362:

<comment>New CJK centering helper is not null-safe and can throw NPE during rendering/formatting.</comment>

<file context>
@@ -329,10 +350,48 @@ public String title(String title, char ch) {
+
+  static int displayWidth(String text) {
+    int width = 0;
+    for (int i = 0; i < text.length(); i++) {
+      char c = text.charAt(i);
+      if (isFullWidth(c)) {
</file context>

[cubic-dev-ai]

P2: I18n uses unsynchronized mutable static locale/bundle state, which is not thread-safe and can leak or mismatch language state across concurrent operations.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At core/src/main/java/org/openapitools/openapidiff/core/output/I18n.java, line 20:

<comment>`I18n` uses unsynchronized mutable static locale/bundle state, which is not thread-safe and can leak or mismatch language state across concurrent operations.</comment>

<file context>
@@ -0,0 +1,72 @@
+  private I18n() {}
+
+  public static void setLocale(Locale locale) {
+    currentLocale = locale;
+    bundle = loadBundle(locale);
+  }
</file context>