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).
1. Check if the import ``import { _t } from 'matrix-react-sdk/src/languageHandler';`` is present. If not add it to the other import statements. Also import `_td` if needed.
1. 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.
1. If you added a string with a plural, you can add other English plural variants to ``src/i18n/strings/en_EN.json`` (remeber to edit the one in the same project as the source file containing your new translation).
1. Decide how to name it. Please think about if the person who has to translate it can understand what it does. E.g. using the name 'recipient' is bad, because a translator does not know if it is the name of a person, an email address, a user ID, etc. Rather use e.g. recipientEmailAddress.
1. Add it to the array in ``_t`` for example ``_t(STRING, {variable: this.variable})``
1. 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.
- You can use the special ``count`` variable to choose between multiple versions of the same string, in order to get the correct pluralization. E.g. ``_t('You have %(count)s new messages', { count: 2 })`` would show 'You have 2 new messages', while ``_t('You have %(count)s new messages', { count: 1 })`` would show 'You have one new message' (assuming a singular version of the string has been added to the translation file. See above). Passing in ``count`` is much prefered over having an if-statement choose the correct string to use, because some languages have much more complicated plural rules than english (e.g. they might need a completely different form if there are three things rather than two).
- If you want to translate text that includes e.g. hyperlinks or other HTML you have to also use tag substitution, e.g. ``_t('<a>Click here!</a>', {}, { 'a': (sub) => <a>{sub}</a> })``. If you don't do the tag substitution you will end up showing literally '<a>' rather than making a hyperlink.
- You can also use React components with normal variable substitution if you want to insert HTML markup, e.g. ``_t('Your email address is %(emailAddress)s', { emailAddress: <i>{userEmailAddress}</i> })``.
- 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.
- Translation 'smell test': If you have a string that does not begin with a capital letter (is not the start of a sentence) or it ends with e.g. ':' or a preposition (e.g. 'to') you should recheck that you are not trying to translate a partial sentence.
- If you have multiple strings, that are almost identical, except some part (e.g. a word or two) it is still better to translate the full sentence multiple times. It may seem like inefficient repetion, but unlike programming where you try to minimize repetition, translation is much faster if you have many, full, clear, sentences to work with, rather than fewer, but incomplete sentence fragments.
