How to translate riot-web (Dev Guide)

Requirements

A working Development Setup
Be able to understand English
Be able to understand the language you want to translate riot-web into

Translating strings vs. marking strings for translation

Translating strings are done with the _t() function found in matrix-react-sdk/lib/languageHandler.js. It is recommended to call this function wherever you introduce a string constant which should be translated. However, translating can not be performed until after the translation system has been initialized. Thus, sometimes translation must be performed at a different location in the source code than where the string is introduced. This breaks some tooling and makes it difficult to find translatable strings. Therefore, there is the alternative _td() function which is used to mark strings for translation, without actually performing the translation (which must still be performed separately, and after the translation system has been initialized).

Basically, whenever a translatable string is introduced, you should call either _t() immediately OR _td() and later _t().

Example:

// Module-level constant
const COLORS = {
    '#f8481c': _td('reddish orange'), // Can't call _t() here yet
    '#fc2647': _td('pinky red') // Use _td() instead so the text is picked up for translation anyway
}

// Function that is called some time after i18n has been loaded
function getColorName(hex) {
    return _t(COLORS[hex]); // Perform actual translation here
}

Adding new strings

Check if the import import { _t } from 'matrix-react-sdk/lib/languageHandler'; is present. If not add it to the other import statements. Also import _td if needed.
Add _t() to your string. (Don't forget curly braces when you assign an expression to JSX attributes in the render method). If the string is introduced at a point before the translation system has not yet been initialized, use _td() instead, and call _t() at the appropriate time.
Add the String to the en_EN.json file in src/i18n/strings (respect which repository you are on).

Adding variables inside a string.

Extend your _t() call. Instead of _t(STRING) use _t(STRING, {})
Decide how to name it. Please think about if the person who has to translate it can understand what it does.
Add it to the array in _t for example _t(STRING, {variable: this.variable})
Add the variable inside the string. The syntax for variables is %(variable)s. Please note the s at the end. The name of the variable has to match the previous used name.

Things to know/Style Guides

Do not use _t() inside getDefaultProps: the translations aren't loaded when getDefaultProps is called, leading to missing translations. Use _td() to indicate that _t() will be called on the string later.
If using translated strings as constants, translated strings can't be in constants loaded at class-load time since the translations won't be loaded. Mark the strings using _td() instead and perform the actual translation later.
If a string is presented in the UI with punctuation like a full stop, include this in the translation strings, since punctuation varies between languages too.
Avoid "translation in parts", i.e. concatenating translated strings or using translated strings in variable substitutions. Context is important for translations, and translating partial strings this way is simply not always possible.
Concatenating strings often also introduces an implicit assumption about word order (e.g. that the subject of the sentence comes first), which is incorrect for many languages.

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