Zoran Luledzija
Zoran Luledzija
May 05, 2023
6 min read
May 05, 2023
6 min read

The role of Accept-Language header in software localization

The role of Accept-Language header in software localization

Ever wondered how websites know your language preferences without asking? It's all thanks to the Accept-Language header! In this blog post, we'll explore what the Accept-Language header is, its purpose, why it was created, and some additional information that will help you understand this often-overlooked aspect of web browsing. So sit back, relax, and let's dive into the world of the Accept-Language header together!

Understanding the Accept-Language header

The Accept-Language header is a part of the HTTP (Hypertext Transfer Protocol) requests that your web browser sends to a website's server when you access a webpage. Its primary function is to communicate your preferred language(s) for viewing content, enabling the server to deliver content in your chosen language if it's available. This clever feature was designed to enhance your web browsing experience by making it smoother and more personalized, ensuring content is presented in the language you're most comfortable with whenever possible.

The main aim of the Accept-Language header is to express a user's language preferences, but this information can be employed in different ways. Consider the following examples of the most frequent situations where the Accept-Language header is utilized:

  • Content localization: By helping websites display content in the user's preferred language, the Accept-Language header fosters a more personalized and enjoyable browsing experience.

  • Language-specific features: The header assists websites in offering language-specific features or promotions by determining which users should be exposed to such content.

  • Analytics and statistics: The Accept-Language header is a valuable tool for website owners to analyze their users' linguistic preferences. This data can inform decisions about which languages to support and prioritize for translation or other business-related choices.

Accept-Language header structure

The Accept-Language header structure might seem a bit confusing at first, but fear not! After reading this chapter, you'll find that it's not as complex as it appears. All you need is a clear understanding of its structure. So let's dive right in!

The Accept-Language header is made up of one or more language tags. Each tag can be followed by an optional 'quality value' or 'q-value' (a value between 0 and 1), which signifies the user's preference level for a specific language – essentially, it serves as a weighting factor or language priority. When a q-value is provided, it is separated from the language tag by a semicolon. If the Accept-Language header includes multiple languages, they are separated by commas.

Accept-Language: en-US, en;q=0.9, es;q=0.8

When examining the Accept-Language headers sent from various browsers or clients to the server, you'll find that their structure remains quite consistent. While the structure of the Accept-Language header stays constant, the specific languages included in the header can differ. These variations are determined by the language preferences of the client initiating the HTTP request to the server.

To check this for yourself, simply set your preferred languages in your browser settings. Then, open some website and inspect the Accept-Language header your browser sends. You can find this information in the Network tab within the Developer Tools of your browser. The image below shows an example of the Accept-Language header sent when the browser's preferred languages are set to English (United States), English, and Spanish.

Browser network

Accept-Language header examples

Let's take a closer look at several Accept-Language header examples and explore the details of each:

  • Accept-Language: en - This indicates that the user prefers content in English.

  • Accept-Language: en-US, en;q=0.9 - This suggests that the user prefers content in US English and English (in general). The server can use this information to prioritize providing content in US English, and if it's unavailable, it may choose the more general English form.

  • Accept-Language: en-US, en;q=0.9, fr-FR;q=0.8, fr;q=0.7 - This implies that the user prefers content in a prioritized order of languages, starting with US English, followed by English in general (q=0.9), French from France (q=0.8), and lastly, French in general (q=0.7). The server can use this information to deliver content that best matches the user's language preferences, beginning with the highest priority option and moving down the list if the preferred language is unavailable.

  • Accept-Language: * - This indicates that the user is open to receiving content in any language. The server can use this information to provide content in the default or most appropriate language based on its own criteria or the user's geographical location.

Remember, the server isn't obligated to strictly follow the Accept-Language header value. Instead, it serves as a helpful suggestion, guiding the server to better understand a user's language preferences and attempting to provide the most suitable content. Nevertheless, the server ultimately makes the final decision on which language version to deliver, considering factors like content availability, server configuration, or other constraints.

Language detection with the Accept-Language header

As we've mentioned earlier in this post, the primary purpose of the Accept-Language header is to communicate the user's preferred languages to the server. However, the final decision on which language version to deliver ultimately lies with the server, taking into account factors such as content availability, server configuration, and other considerations.

The Accept-Language header plays a significant role in software localization, making the use of software products more enjoyable by adapting content to the user's preferred languages. Moreover, since detecting user preferences is crucial for delivering appropriate content, we encourage you to explore another one of our blog posts that explains how to detect the navigator language to serve the most suitable version of the content.

Internationalizing REST APIs with the Accept-Language header

Localization is crucial for both webpages and REST APIs, as many software applications depend on them and frequently display the acquired data directly. You may have unknowingly used localized REST APIs in various global markets, such as e-commerce and mapping applications. Furthermore, you might have come across websites where most content is in your preferred language, but some information retrieved from non-localized third-party REST APIs appears in an entirely different language. By localizing REST APIs, these inconsistencies can be resolved, ensuring a seamless user experience.

The process of localizing REST APIs is similar to localizing websites. It involves extracting and parsing the Accept-Language header, identifying the best language match, loading the relevant resources for the chosen language, and serving content in that language to the caller.

However, it's important to be careful when using the Accept-Language header while designing REST APIs. Its primary function is to communicate a user's preferred language to the server. In some cases, REST APIs might include locale-sensitive business rules where using a URL or body parameter for a specific region is more appropriate. Therefore, carefully considering all aspects when designing internationalized REST APIs is crucial.

Setting the Accept-Language header in JavaScript

In this section, we will demonstrate a simple example of making an AJAX call using JavaScript's fetch function, along with setting a custom Accept-Language header. Sometimes, this header may be automatically included for you. However, this example shows how effortless it is to set or overwrite the Accept-Language header when needed.

fetch('https://example.com/api/data', {
  method: 'GET',
  headers: {
    'Accept-Language': 'de-DE' // Custom Accept-Language header
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error fetching data:', error));


The Accept-Language header is a powerful yet easy-to-understand tool for software localization. It allows websites and REST APIs to provide content tailored to users' preferred languages, ensuring a smooth and enjoyable browsing experience for everyone. By understanding its structure and applying it effectively, developers can create accessible applications for the global market.

Facing the challenges of software localization? Give the Localizely platform a try! It streamlines the process, making it easy for you to handle translations and work closely with your team in a more efficient manner.

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Zoran Luledzija
Zoran Luledzija

Zoran is a Software Engineer at Localizely. His primary interest is web development, but he also has a solid background in other technologies. For the last few years, he has been involved in the development of software localization tools.

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