Internationalization for developers

Zulip is designed with internationalization (i18n) in mind, which lets users view the Zulip UI in their preferred language. As a developer, it’s your responsibility to make sure that:

UIs you implement look good when translated into languages other than English.
Any strings your code changes touch are correctly marked for translation.

This pages gives concrete guidance on how to accomplish these goals, as well as providing additional context for those who are curious.

How internationalization impacts Zulip’s UI

Always be mindful that text width is not a constant. The width of the string needed to express something varies dramatically between languages. This means you can’t just hardcode a button or widget to look great for English and expect it to work in all languages.

You can test your work by changing the lengths of strings to be 50% longer and 50% shorter than in English. For strings that are already in the Zulip UI, Russian is a good test case for translations that are generally longer than English. Japanese translations are generally shorter.

What should be marked for translation

Our goal is for all user-facing strings in Zulip to be tagged for translation in both HTML templates and code, and our linters attempt to enforce this. This applies to every bit of language a user might see, including things like error strings, dates, and email content.

The exceptions to the “tag everything users sees” rule are:

Landing pages (e.g., https://zulip.com/features/)
Help center pages
Zulip updates

We do aim for those pages to be usable with tools like Google Translate.

Note that the “user-facing” part is also important. To make good use of our community translators’ valuable time, we only tag content that will actually be displayed to users.

How to mark a string for translation

When tagging strings for translation, variation between languages means that you have to be careful in exactly what you tag, and how you split things up:

Punctuation varies between languages (e.g., Japanese doesn’t use .s at the end of sentences). This means that you should always include end-of-sentence symbols like . and ? inside the to-be-translated strings, so that translators can correctly translate the content.
Word order varies between languages (e.g., some languages put subjects before verbs, others the other way around). This means that concatenating translatable strings produces broken results. If a sentence contains a variable, never tag the part before the variable separately from the part after the variable.
Strings with numerals (e.g., “5 bananas”) work quite differently between languages, so double-check your work when tagging strings with numerals for translation. See the plurals section below for details.

Note also that we have a “sentence case” capitalization policy that we enforce using linters that check all strings tagged for translation in Zulip.

Translation syntax in Zulip

A few general notes:

Translation functions must be passed the string to translate, not a variable containing the target string. Otherwise, the parsers that extract the strings in a project to send to translators will not find your string.
Zulip makes use of the Jinja2 templating system for the server and Handlebars for the web app. Our HTML templates documentation includes useful information on the syntax and behavior of these systems.

Web application translations

We use the FormatJS library for translations in the Zulip web app, both in Handlebars templates and JavaScript.

FormatJS uses the standard ICU MessageFormat, which includes useful features such as plural translations.

To mark a string translatable in JavaScript files, pass it to the intl.formatMessage function, which we alias to $t in intl.js:

$t({defaultMessage: "English text"})

The string to be translated must be a constant literal string, but variables can be interpolated by enclosing them in braces (like {variable}) and passing a context object:

$t({defaultMessage: "English text with a {variable}"}, {variable: "Variable value"})

$t does not escape any variables, so if your translated string is eventually going to be used as HTML, use $t_html instead.

html_content = $t_html({defaultMessage: "HTML with a {variable}"}, {variable: "Variable value"});
$("#foo").html(html_content);

The only HTML tags allowed directly in translated strings are the simple HTML tags enumerated in default_html_elements (web/src/i18n.ts) with no attributes. This helps to avoid exposing HTML details to translators. If you need to include more complex markup such as a link, you can define a custom HTML tag locally to the translation, or use a Handlebars template:

$t_html(
    {defaultMessage: "<b>HTML</b> linking to the <z-link>login page</z-link>"},
    {"z-link": (content_html) => `<a href="/login/">${content_html.join("")}</a>`},
)

Plurals and lists

Plurals are a complex detail of human language. In English, there are only two variants for how a word like “banana” might be spelled depending on the number of objects being discussed: “1 banana” and “2 bananas”. But languages vary greatly in how plurals work. For example, in Russian, the form a noun takes depends in part on the last digit of its quantity.

To solve this problem, Zulip expresses plural strings using the standard ICU MessageFormat syntax, which defines how the string varies depending on whether there’s one item or many in English:

"{N, plural, one {Done! {N} message marked as read.} other {Done! {N} messages marked as read.}}"

Translators are then able to write a translation using this same syntax, potentially using a different set of cases, like this Russian translation, which varies the string based on whether there was 1, few, or many items:

"{N, plural, one {Готово! {N} сообщение помечено как прочитанное.} few {Готово! {N} сообщений помечены как прочитанные.} many {Готово! {N} сообщений помечены как прочитанные.} other {Готово! {N} сообщений помечены как прочитанные.}}"

You don’t need to understand how to write Russian plurals. As a developer, you just need to write the correct ICU plurals for English, which will always just have singular and plural variants, and translators can take care of the rest.

Nonetheless, even the English format takes some concentration to read. So when designing UI, we generally try to avoid unnecessarily writing strings that require plurals in favor of other ways to present information, like displaying an icon with a number next to it.

Languages differ greatly in how to construct a list of the form “foo, bar, and baz”. Some languages don’t use commas! The web application has a handy util.format_array_as_list function for correctly doing this using the Intl module; use git grep to find examples.

Handlebars templates

For translations in Handlebars templates we also use FormatJS, through two Handlebars helpers that Zulip registers. The syntax for simple strings is:

{{t 'English text' }}

{{t 'Block of English text with a {variable}.' }}

If you are passing a translated string to a Handlebars partial, you can use:

{{> template_name
    variable_name=(t 'English text')
    }}

The syntax for HTML strings is:

{{#tr}}
    <p>Block of English text.</p>
{{/tr}}

{{#tr}}
    <p>Block of English text with a {variable}.</p>
{{/tr}}

Just like in JavaScript code, variables are enclosed in single braces (rather than the usual Handlebars double braces). Unlike in JavaScript code, variables are automatically escaped by our Handlebars helper.

Handlebars expressions like {{variable}} or blocks like {{#if}}...{{/if}} aren’t permitted inside a {{#tr}}...{{/tr}} translated block, because they don’t work properly with translation. The Handlebars expression would be evaluated before the string is processed by FormatJS, so that the string to be translated wouldn’t be constant. We have a linter to enforce that translated blocks don’t contain Handlebars.

Restrictions on including HTML tags in translated strings are the same as in JavaScript. You can insert more complex markup using a local custom HTML tag like this:

{{#tr}}
    <b>HTML</b> linking to the <z-link>login page</z-link>
    {{#*inline "z-link"}}<a href="/login/">{{> @partial-block}}</a>{{/inline}}
{{/tr}}

Server translations

Strings in the server primarily comprise two areas:

Error strings and other values returned by the API.
Strings in portico pages, such as the login flow, that are not rendered using JavaScript or Handlebars.

Jinja2 templates

All user-facing text in the Zulip UI should be generated by an Jinja2 HTML template so that it can be translated.

To mark a string for translation in a Jinja2 template, you can use the _() function in the templates like this:

{{ _("English text") }}

If a piece of text contains both a literal string component and variables, use a block translation. This puts in placeholders for variables, to allow translators to translate an entire sentence.

To tag a block for translation, Jinja2 uses the trans tag, like this:

{% trans %}This string will have {{ value }} inside.{% endtrans %}

Never break up a sentence like this, as it will make it impossible to translate correctly:

# Don't do this!
{{ _("This string will have") }} {{ value }} {{ _("inside") }}

Python

A string in Python can be marked for translation using the _() function, which can be imported as follows:

from django.utils.translation import gettext as _

Zulip expects all the error messages to be translatable as well. To ensure this, the error message passed to JsonableError should always be a literal string enclosed by _() function, for example:

JsonableError(_('English text'))

If you’re declaring a user-facing string at top level or in a class, you need to use gettext_lazy instead, to ensure that the translation happens at request-processing time when Django knows what language to use, for example:

from zproject.backends import check_password_strength, email_belongs_to_ldap

AVATAR_CHANGES_DISABLED_ERROR = gettext_lazy("Avatar changes are disabled in this organization.")

def confirm_email_change(request: HttpRequest, confirmation_key: str) -> HttpResponse:
  ...

class Realm(models.Model):
    MAX_REALM_NAME_LENGTH = 40
    MAX_REALM_SUBDOMAIN_LENGTH = 40

    ...
    ...

    STREAM_EVENTS_NOTIFICATION_TOPIC = gettext_lazy("channel events")

To ensure we always internationalize our JSON error messages, the Zulip linter (tools/lint) attempts to verify correct usage.

Translation process

The end-to-end tooling process for translations in Zulip is as follows.

The strings are marked for translation (see sections for server and web app translations for details on this).
Translation resource files are created using the ./manage.py makemessages command. This command will create, for each language, a resource file called translations.json for the web app strings and django.po for the server strings.

The makemessages command is idempotent in that:
- It will only delete singular keys in the resource file when they are no longer used in Zulip code.
- It will only delete plural keys (see above for the documentation on plural translations) when the corresponding singular key is absent.
- It will not override the value of a singular key if that value contains a translated text.
Those resource files are uploaded to Transifex by a maintainer using the ./tools/i18n/push-translations command (which invokes a Transifex API tool, tx push, internally).
Translators translate the strings in the Transifex UI. (In theory, it’s possible to translate locally and then do tx push, but because our workflow is to sync translation data from Transifex to Zulip, making changes to translations in Zulip risks having the changes blown away by a data sync, so that’s only a viable model for a language that has no translations yet).
The translations are downloaded back into the codebase by a maintainer, using tools/i18n/sync-translations (which invokes the Transifex API tool, tx pull, internally).

If you’re interested, you may also want to check out the translators’ workflow, just so you have a sense of how everything fits together.

Translation resource files

All the translation magic happens through resource files, which hold the translated text. Server resource files are located at locale/<lang_code>/LC_MESSAGES/django.po, while web app resource files are located at locale/<lang_code>/translations.json (and mobile at mobile.json).

These files are uploaded to Transifex, where they can be translated.

Working with Transifex

Transifex config

The config file that maps the resources from Zulip to Transifex is located at .tx/config.

Transifex CLI setup

In order to be able to run tx pull (and tx push as well, if you’re a maintainer), you have to specify your Transifex API Token, generated in Transifex’s web interface, in a config file located at ~/.transifexrc.

You can find details on how to set it up here, but it should look similar to this (with your credentials):

[https://www.transifex.com]
rest_hostname = https://rest.api.transifex.com
token = 1/abcdefg...

This basically identifies you as a Transifex user, so you can access your organizations from the command line.

Additional resources

We recommend the EdX i18n guide as a great resource for learning more about internationalization in general; we agree with essentially all of their style guidelines.