Arguments in a label: sprintf and ICU plurals 

Plurals and selections with ICU MessageFormat 

ICU MessageFormat picks one wording by a value. Use it where a count or a gender changes the sentence, because the rules for that differ per language. A German label needs two forms for a count, a Polish one needs four, and sprintf() cannot express either.

Write the pattern into the label as it stands:

<?xml version="1.0" encoding="UTF-8"?>
<xliff version="1.2" xmlns="urn:oasis:names:tc:xliff:document:1.2">
  <file source-language="en" datatype="plaintext" original="locallang.xlf">
    <header/>
    <body>
      <trans-unit id="file_count">
        <source>{count, plural, one {# file} other {# files}}</source>
      </trans-unit>
      <trans-unit id="item_count">
        <source>{count, plural, =0 {no items} one {# item} other {# items}}</source>
      </trans-unit>
      <trans-unit id="greeting">
        <source>Hello {name}, you have {count, plural, one {# message} other {# messages}}.</source>
      </trans-unit>
      <trans-unit id="profile_update">
        <source>{gender, select, male {He} female {She} other {They}} updated the profile.</source>
      </trans-unit>
      <trans-unit id="welcome">
        <source>Welcome, {name}!</source>
      </trans-unit>
    </body>
  </file>
</xliff>

{count, plural, …}

Chooses by the plural category of the value: one, other, and the categories of the current language. =0 matches exactly zero, which gives a sentence like "no items" its own wording. A # inside a branch prints the number.

{gender, select, …}

Chooses by an exact value. other catches the rest.

{name}

Prints the value.

Then pass the values under their names:

$this->getTranslator()->label(
    'my_extension.messages:file_count',
    ['count' => 5],
);

<f:translate key="my_extension.messages:file_count"
    arguments="{count: files}" />

TYPO3 reads the plural rules of the current locale, and falls back to en_US where it knows no locale. So the same label gives "5 files" in English and "5 Dateien" in German, each with the forms of that language.

Where a pattern is invalid, TYPO3 returns the label unchanged, with the braces still in it. A sentence of curly braces on a page is the sign to look for a typo in the pattern.

Positional values with sprintf() 

A list of values keeps the behavior that TYPO3 always had: the label is a sprintf() format string.

$this->getTranslator()->label(
    'my_extension.messages:greeting_sprintf',
    ['World'],
);

TYPO3 uses vsprintf() here, which reports a count of values that does not match the count of placeholders. A label with one %s and no value therefore raises an error instead of printing an empty string.

Do not mix the two. A label written for sprintf() keeps its %s untouched when the call passes named values. An ICU pattern reaches the output with its braces when the call passes a list.