Add documentation about extension localization

.github/CONTRIBUTING.md

@@ -96,6 +96,39 @@ WebExtensions message format, with a few differences:
 * Referencing other messages is possible, include a placeholder to another key in the placeholder content.
 * Limited pluralization is possible, supporting a very basic subset of the ICU message pluralization format.
 
+The pluralization support is based on a very basic subset of the ICU message
+pluralization syntax, with the following limitations:
+
+* Supports only the pluralization type (`plural`).
+* Supports only one variable key.
+* Does not support nested messages.
+* Only supports `#` placeholder inside messages.
+
+As Crowdin is compatible with this format, you mostly don't need to worry about
+these changes if you are translating through Crowdin. More details about special
+features can be found [in a Crowdin discussion](https://crowdin.com/project/firefoxpwa/discussions/2).
+
+<details>
+ <summary>Notes for developers</summary>
+
+The localization messages are stored in a WebExtensions-compatible format.
+When creating a new message, please make sure that it follows the same
+message ID style as other messages and contains an appropriate description.
+
+When creating an HTML element that needs to be translated, add a `data-i18n`
+attribute with its content set to the message ID. If you need to translate
+specific attributes, you can set `data-i18n-ATTRIBUTE-NAME` to the message
+ID. Even if the element only uses attribute translations, it still needs
+a `data-i18n` attribute, but it can be empty.
+
+For accessing translated messages through JS, the `getMessage` function
+from the `i18n` file can be used.
+
+For accessing messages from the background script, use the standard
+`browser.i18n.getMessage` function instead. In this case, make sure
+to add the uses message IDs to the `messages` list in `package.json`.
+</details>
+
 ### UserChrome Translations
 
 It is currently not possible to translate the UserChrome messages, but support

README.md

@@ -81,7 +81,7 @@ Thanks to [all contributors](https://github.com/filips123/PWAsForFirefox/graphs/
 
 ### Other Mentions
 
-Thanks to [all package maintainers](https://repology.org/project/firefoxpwa/information) making sure the project is up-to-date! Thanks to [all stargazers](https://github.com/filips123/PWAsForFirefox/stargazers) who starred our repository on GitHub. Finally, thanks to Mozilla and its developers for creating Firefox and making it possible to modify its UI using JavaScript!
+Thanks to [all package maintainers](https://repology.org/project/firefoxpwa/information) making sure the project is up-to-date! Thanks to [all translators](https://crowdin.com/project/firefoxpwa) making the project available in multiple languages! Thanks to [all stargazers](https://github.com/filips123/PWAsForFirefox/stargazers) who starred our repository on GitHub. Finally, thanks to Mozilla and its developers for creating Firefox and making it possible to modify its UI using JavaScript!
 
 ## Versioning
 

docs/docs/index.md

@@ -115,7 +115,8 @@ to this project for providing help and developing features!
 <h3>Other Mentions</h3>
 
 Thanks to [all package maintainers](https://repology.org/project/firefoxpwa/information)
-making sure the project is up-to-date! Thanks to [all stargazers](https://github.com/filips123/PWAsForFirefox/stargazers)
+making sure the project is up-to-date! Thanks to [all translators](https://crowdin.com/project/firefoxpwa)
+making the project available in multiple languages! Thanks to [all stargazers](https://github.com/filips123/PWAsForFirefox/stargazers)
 who starred our repository on GitHub. Finally, thanks to Mozilla and its developers for
 creating Firefox and making it possible to modify its UI using JavaScript!
 

docs/docs/user-guide/extension.md

@@ -171,6 +171,16 @@ when a website has own "redirection" URLs that you do not want to open in a web
 This option only has an effect when [automatic web app launching](#enable-automatic-web-app-launching)
 is enabled.
 
+### Language
+
+The extension is translated into multiple languages. You can select which one you want to use.
+
+!!! warning
+
+ Translations are maintained by the community and may be incomplete or incorrect.
+
+ You can help to improve them on [Crowdin](https://crowdin.com/project/firefoxpwa).
+
 ### Update web apps
 
 This will download and parse manifests for all your web apps and register them again to