How to Translate Messages using the ICU MessageFormat

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How to Translate Messages using the ICU MessageFormat

Messages (i.e. strings) in applications are almost never completely static. They contain variables or other complex logic like pluralization. To handle this, the Translator component supports the ICU MessageFormat syntax.

Tip

You can test out examples of the ICU MessageFormatter in this online editor.

Using the ICU Message Format

In order to use the ICU Message Format, the message domain has to be suffixed with +intl-icu:

Normal file name ICU Message Format filename
messages.en.yaml messages+intl-icu.en.yaml
messages.fr_FR.xlf messages+intl-icu.fr_FR.xlf
admin.en.yaml admin+intl-icu.en.yaml

All messages in this file will now be processed by the MessageFormatter during translation.

Message Placeholders

The basic usage of the MessageFormat allows you to use placeholders (called arguments in ICU MessageFormat) in your messages:

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# translations/messages+intl-icu.en.yaml
say_hello: 'Hello {name}!'

Caution

In the previous translation format, placeholders were often wrapped in % (e.g. %name%). This % character is no longer valid with the ICU MessageFormat syntax, so you must rename your parameters if you are upgrading from the previous format.

Everything within the curly braces ({...}) is processed by the formatter and replaced by its placeholder:

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// prints "Hello Fabien!"
echo $translator->trans('say_hello', ['name' => 'Fabien']);

// prints "Hello Symfony!"
echo $translator->trans('say_hello', ['name' => 'Symfony']);

Selecting Different Messages Based on a Condition

The curly brace syntax allows to "modify" the output of the variable. One of these functions is the select function. It acts like PHP's switch statement and allows you to use different strings based on the value of the variable. A typical usage of this is gender:

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# translations/messages+intl-icu.en.yaml

# the 'other' key is required, and is selected if no other case matches
invitation_title: >-
    {organizer_gender, select,
        female   {{organizer_name} has invited you to her party!}
        male     {{organizer_name} has invited you to his party!}
        multiple {{organizer_name} have invited you to their party!}
        other    {{organizer_name} has invited you to their party!}
    }

This might look very complex. The basic syntax for all functions is {variable_name, function_name, function_statement} (where, as you see later, function_statement is optional for some functions). In this case, the function name is select and its statement contains the "cases" of this select. This function is applied over the organizer_gender variable:

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// prints "Ryan has invited you to his party!"
echo $translator->trans('invitation_title', [
    'organizer_name' => 'Ryan',
    'organizer_gender' => 'male',
]);

// prints "John & Jane have invited you to their party!"
echo $translator->trans('invitation_title', [
    'organizer_name' => 'John & Jane',
    'organizer_gender' => 'multiple',
]);

// prints "ACME Company has invited you to their party!"
echo $translator->trans('invitation_title', [
    'organizer_name' => 'ACME Company',
    'organizer_gender' => 'not_applicable',
]);

The {...} syntax alternates between "literal" and "code" mode. This allows you to use literal text in the select statements:

  1. The first {organizer_gender, select, ...} block starts the "code" mode, which means organizer_gender is processed as a variable.
  2. The inner {... has invited you to her party!} block brings you back in "literal" mode, meaning the text is not processed.
  3. Inside this block, {organizer_name} starts "code" mode again, allowing organizer_name to be processed as a variable.

Tip

While it might seem more logical to only put her, his or their in the switch statement, it is better to use "complex arguments" at the outermost structure of the message. The strings are in this way better readable for translators and, as you can see in the multiple case, other parts of the sentence might be influenced by the variables.

Tip

It's possible to translate ICU MessageFormat messages directly in code, without having to define them in any file:

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$invitation = '{organizer_gender, select,
    female   {{organizer_name} has invited you to her party!}
    male     {{organizer_name} has invited you to his party!}
    multiple {{organizer_name} have invited you to their party!}
    other    {{organizer_name} has invited you to their party!}
}';

// prints "Ryan has invited you to his party!"
echo $translator->trans(
    $invitation,
    [
        'organizer_name' => 'Ryan',
        'organizer_gender' => 'male',
    ],
    // if you prefer, the required "+intl-icu" suffix is also defined as a constant:
    // Symfony\Component\Translation\MessageCatalogueInterface::INTL_DOMAIN_SUFFIX
    'messages+intl-icu'
);

Pluralization

Another interesting function is plural. It allows you to handle pluralization in your messages (e.g. There are 3 apples vs There is one apple). The function looks very similar to the select function:

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# translations/messages+intl-icu.en.yaml
num_of_apples: >-
    {apples, plural,
        =0    {There are no apples}
        one   {There is one apple...}
        other {There are # apples!}
    }

Pluralization rules are actually quite complex and differ for each language. For instance, Russian uses different plural forms for numbers ending with 1; numbers ending with 2, 3 or 4; numbers ending with 5, 6, 7, 8 or 9; and even some exceptions to this!

In order to properly translate this, the possible cases in the plural function are also different for each language. For instance, Russian has one, few, many and other, while English has only one and other. The full list of possible cases can be found in Unicode's Language Plural Rules document. By prefixing with =, you can match exact values (like 0 in the above example).

Usage of this string is the same as with variables and select:

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// prints "There is one apple..."
echo $translator->trans('num_of_apples', ['apples' => 1]);

// prints "There are 23 apples!"
echo $translator->trans('num_of_apples', ['apples' => 23]);

Note

You can also set an offset variable to determine whether the pluralization should be offset (e.g. in sentences like You and # other people / You and # other person).

Tip

When combining the select and plural functions, try to still have select as outermost function:

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{gender_of_host, select,
    female {{num_guests, plural, offset:1
        =0    {{host} does not give a party.}
        =1    {{host} invites {guest} to her party.}
        =2    {{host} invites {guest} and one other person to her party.}
        other {{host} invites {guest} and # other people to her party.}
    }}
    male {{num_guests, plural, offset:1
        =0    {{host} does not give a party.}
        =1    {{host} invites {guest} to his party.}
        =2    {{host} invites {guest} and one other person to his party.}
        other {{host} invites {guest} and # other people to his party.}
    }}
    other {{num_guests, plural, offset:1
        =0    {{host} does not give a party.}
        =1    {{host} invites {guest} to their party.}
        =2    {{host} invites {guest} and one other person to their party.}
        other {{host} invites {guest} and # other people to their party.}
    }}
}

Using Ranges in Messages

The pluralization in the legacy Symfony syntax could be used with custom ranges (e.g. have different messages for 0-12, 12-40 and 40+). The ICU message format does not have this feature. Instead, this logic should be moved to PHP code:

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// Instead of
$message = $translator->trans('balance_message', $balance);
// with a message like:
// ]-Inf,0]Oops! I'm down|]0,1000]I still have money|]1000,Inf]I have lots of money

// use three different messages for each range:
if ($balance < 0) {
    $message = $translator->trans('no_money_message');
} elseif ($balance < 1000) {
    $message = $translator->trans('some_money_message');
} else {
    $message = $translator->trans('lots_of_money_message');
}

Additional Placeholder Functions

Besides these, the ICU MessageFormat comes with a couple other interesting functions.

Ordinal

Similar to plural, selectordinal allows you to use numbers as ordinal scale:

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# translations/messages+intl-icu.en.yaml
finish_place: >-
    You finished {place, selectordinal,
        one   {#st}
        two   {#nd}
        few   {#rd}
        other {#th}
    }!

# when only formatting the number as ordinal (like above), you can also
# use the `ordinal` function:
finish_place: You finished {place, ordinal}!
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// prints "You finished 1st!"
echo $translator->trans('finish_place', ['place' => 1]);

// prints "You finished 9th!"
echo $translator->trans('finish_place', ['place' => 9]);

// prints "You finished 23rd!"
echo $translator->trans('finish_place', ['place' => 23]);

The possible cases for this are also shown in Unicode's Language Plural Rules document.

Date and Time

The date and time function allows you to format dates in the target locale using the IntlDateFormatter:

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# translations/messages+intl-icu.en.yaml
published_at: 'Published at {publication_date, date} - {publication_date, time, short}'

The "function statement" for the time and date functions can be one of short, medium, long or full, which correspond to the constants defined by the IntlDateFormatter class:

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// prints "Published at Jan 25, 2019 - 11:30 AM"
echo $translator->trans('published_at', ['publication_date' => new \DateTime('2019-01-25 11:30:00')]);

Numbers

The number formatter allows you to format numbers using Intl's NumberFormatter:

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# translations/messages+intl-icu.en.yaml
progress: '{progress, number, percent} of the work is done'
value_of_object: 'This artifact is worth {value, number, currency}'
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// prints "82% of the work is done"
echo $translator->trans('progress', ['progress' => 0.82]);
// prints "100% of the work is done"
echo $translator->trans('progress', ['progress' => 1]);

// prints "This artifact is worth $9,988,776.65"
// if we would translate this to i.e. French, the value would be shown as
// "9 988 776,65 €"
echo $translator->trans('value_of_object', ['value' => 9988776.65]);
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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