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As a developer, have you ever been overwhelmed by these numerous codes?

  • When users register, should the country list use CN or CHN?
  • When doing multilingual translation (i18n), should the folder be named zh or zh-CN?
  • When processing video subtitles, the specification requires an unfamiliar three-letter code, sometimes zho and sometimes chi. What are the differences?
  • Not to mention time zone identifiers like Asia/Shanghai, which seem to have no rules.

After reading this, you will completely understand the logic behind these codes and be confident in using them correctly in your projects.

Core Idea: Divide and Conquer

These standards seem confusing because we're trying to understand them with a vague concept of "region." However, the principle of the computer world is precision. Therefore, international standards organizations "divide and conquer" the vague concept of "region" into several specific, orthogonal (independent) dimensions and establish a golden standard for each dimension.

Our journey of exploration begins with understanding these dimensions.

1. Geographical Location: Where Am I? - ISO 3166-1 ​

This is the foundation of all codes, answering the simplest question: "What country/region is this?"

  • Standard Name: ISO 3166-1
  • Core Task: Provide unique identifiers for countries and regions worldwide.
  • Main Forms:
    • alpha-2 (two-letter code): e.g., US, CN, JP. This is the most common and universal form.
    • alpha-3 (three-letter code): e.g., USA, CHN, JPN. More readable, often used in data statistics and official documents.

Developer Practical Guide: ​

  • Database Design: When storing countries in the user table, create a country_code field and use the CHAR(2) type to store the two-letter code (alpha-2). For example:
sql
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(255),
    country_code CHAR(2)
);
  • API Design: Region-related APIs (such as e-commerce delivery range) should use the two-letter code as a parameter, for example:
http
GET /api/v1/shipping?country=CN HTTP/1.1
  • Frontend Development: In the country selection drop-down box, the value of <option value="CN">中国</option> should use the two-letter code. For example:
html
<select name="country">
    <option value="CN">中国</option>
    <option value="US">美国</option>
    <option value="JP">日本</option>
</select>

Learn More ​

2. Language: What Language Do I Speak? - ISO 639 ​

This standard only cares about one thing: Which language are we using?

  • Standard Name: ISO 639
  • Core Task: Encode the languages of the world.
  • Main Forms:
    • ISO 639-1 (two-letter code): e.g., en, zh, ja. It covers about 184 major languages in the world and is conventionally used in lowercase.
    • ISO 639-2 (three-letter code, divided into T and B categories): e.g., eng, zho, jpn. It covers more than 500 languages, solving the problem of insufficient coverage of two-letter codes.
    • ISO 639-3 (three-letter code): e.g., eng, zho, jpn. ISO 639-3 is an extension of ISO 639-2, aiming to cover the superset of all individual languages.

Learn More ​

3. Precise Localization: Where Am I and What Language Do I Speak? - Locale ​

Now, we combine the previous two to answer a more precise question: "What specific language is used by users in a specific region?" This is the concept of Locale.

  • Standard Name: No single standard, usually follows the IETF BCP 47 specification, which combines ISO 639 and ISO 3166-1.
  • Core Task: Accurately describe the variants of languages in specific regions to handle differences in spelling, wording, date formats, currency symbols, etc.
  • Format: language code-COUNTRY code (language-COUNTRY)
    • en-US: English used in the United States.
    • en-GB: English used in the United Kingdom.
    • zh-CN: Chinese used in mainland China (specifically simplified).
    • zh-TW: Chinese used in Taiwan, China (specifically traditional).

Developer Practical Guide: ​

  • Software Internationalization (i18n): Your resource files (such as translation strings) should be placed in folders named after Locale, such as values-zh-CN/strings.xml (Android). For example:
res/
    values/
        strings.xml
    values-zh-CN/
        strings.xml
  • HTTP Request Header: Parse the Accept-Language: zh-CN,zh;q=0.9 request header to return the most suitable language version for the user. For example:
http
Accept-Language: zh-CN,zh;q=0.9
  • Date/Currency Formatting: Libraries in all modern programming languages ​​accept Locale as a parameter. For example, in Java:
java
Locale locale = new Locale("zh", "CN");
DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.DEFAULT, locale);
String dateStr = dateFormat.format(new Date());

Learn More ​

4. Professional Fields and Special Situations: Subtitles, Multimedia and T/B Codes - ISO 639-2 ​

Why don't video subtitles directly use zh or en? Because professional fields require broader language coverage, and this is the root cause of the "one language, multiple codes" problem.

  • Standard Name: ISO 639-2 (three-letter code)

  • Key Knowledge Point: T/B Codes (Terminology/Bibliographic Codes) About 20 languages ​​have two three-letter codes in ISO 639-2, which stems from historical reasons:

    • B Code (Bibliographic): Derived from the English name, mainly used for library cataloging, it is a legacy product. For example, German -> ger.
    • T Code (Terminology): Derived from the local name of the language, it is the code recommended for use in modern computer applications. For example, Deutsch -> deu.

    The most common example is Chinese:

    • chi is the B code (from Chinese).
    • zho is the T code (from 中文, Zhōngwén).
LanguageEnglish NameLocal NameB Code (Old/Catalog)T Code (New/Terminology)Recommended Use
ChineseChinese中文chizhozho
GermanGermanDeutschgerdeudeu
FrenchFrenchFrançaisfrefrafra
TibetanTibetanབོད་ཡིགtibbodbod

Developer Practical Guide: ​

  • Golden Rule: Prioritize using T codes! It is designed for technical applications. But when dealing with old systems or external data, your code needs to be compatible and be able to recognize both T codes and B codes at the same time.
  • Media Processing: When using FFmpeg, you should use the T code. For example:
bash
ffmpeg -i input.mp4 -metadata:s:s:0 language=zho output.mp4
  • Data Cleaning: When receiving data from external sources, you can use mapping functions to unify the code. For example, in Python:
python
language_map = {
    "chi": "zho",
    "ger": "deu",
    "fre": "fra",
    "tib": "bod",
}

def normalize_language_code(code):
    return language_map.get(code, code)

5. The Ultimate Challenge: Time and Time Zones - IANA Time Zone Database ​

Why can't the country code US be used to represent US time? Because the United States has 4 time zones and involves complex daylight saving time rules.

  • Standard Name: IANA Time Zone Database (also known as tz database or Olson database)
  • Core Task: Accurately define the boundaries of all time zones in the world, the offset from UTC, and all historical daylight saving time change rules.
  • Format: Continent/Representative City (Area/Location)
    • Asia/Shanghai
    • America/New_York
    • Europe/London

Developer Practical Guide: ​

  • Golden Rule: Never calculate time zones or daylight saving time yourself!
  • Backend Development: On the server, all times should be stored in UTC. When converting to local time, use the IANA identifier. For example, in Java:
java
Instant instant = Instant.now();
String timestamp = instant.toString();
  • Frontend Development: The browser API can get the user's time zone. For example, in JavaScript:
javascript
const timeZone = Intl.DateTimeFormat().resolvedOptions().timeZone;

Learn More ​

Quick Reference Cheat Sheet ​

Task ScenarioWhat Do I Need?Using StandardExample CodeDeveloper Key Points
Select CountryUniquely identify a countryISO 3166-1 alpha-2CN, USDatabase CHAR(2) storage, API parameters
Webpage or Simple TranslationIdentify a major languageISO 639-1zh, enHTML lang attribute, i18n base
Precise LocalizationDistinguish regional variations of languageIETF BCP 47zh-CN, en-USi18n folder naming, HTTP request header, formatting
Subtitle/Audio Track MarkingCover as many languages ​​as possibleISO 639-2zho (recommended)Prioritize T codes, compatible with B codes
Handle Local TimePrecisely calculate time and daylight saving timeIANA Time Zone DBAsia/ShanghaiServer stores UTC, client uses IANA identifier for conversion

Now, the mist has cleared. These codes are not the product of confusion, but a well-designed, clearly divided system. Master them, and you will be able to:

  1. Establish a Clear Mental Model: Understand the applicable scenarios of each code and understand the historical reasons for special situations such as zho/chi.
  2. Write More Robust Code: Elegantly handle global user needs while being compatible with old data.
  3. Collaborate Efficiently: Communicate with the team using precise terminology.