Catalog formats

Catalog format (configured by format option) refers to file format of offline catalog. This format is never used in production, because it’s compiled into JS module. The reason behind this build step is that choice of catalog format depends on individual internationalization workflow. On the other hand runtime catalog should be as simple as possible so it parsed quickly without additional overhead.

PO File with gettext Plurals

When using localization backends that don’t understand the ICU plural syntax exported by the default po formatter, po-gettext can be used to read and write to PO files using gettext-native plurals.

This is how the regular PO format exports plurals:

msgid "{count, plural, one {Message} other {Messages}}"
msgstr "{count, plural, one {Message} other {Messages}}"

With po-gettext, plural messages are exported in the following way, depending on wheter an explicit ID is set:

# Message with custom ID "my_message" that is pluralized on property "someCount".
# Notice that 'msgid_plural' was generad by appending a '_plural' suffix.
#. js-lingui:pluralize_on=someCount
msgid "my_message"
msgid_plural "my_message_plural"
msgstr[0] "Singular case"
msgstr[1] "Case number {someCount}"

# Message without custom ID that is pluralized on property "anotherCount".
# Notice how 'msgid' and 'msgid_plural' were extracted from original message.
# To allow matching this PO item to the appropriate catalog entry when deserializing,
# the original ICU message is also stored in the generated comment.
#. js-lingui:icu=%7BanotherCount%2C+plural%2C+one+%7BSingular+case%7D+other+%7BCase+number+%7BanotherCount%7D%7D%7D&pluralize_on=anotherCount
msgid "Singular case"
msgid_plural "Case number {anotherCount}"
msgstr[0] "Singular case"
msgstr[1] "Case number {anotherCount}"

Note that this format comes with several caveats and should therefore only be used if using ICU plurals in PO files is not an option:

  • Nested/multiple plurals in one message as shown in plural are not supported as they cannot be expressed with gettext plurals. Messages containing nested/multiple formats will not be output correctly.

  • select and selectOrdinal cannot be expressed with gettext plurals, but the original ICU format is still saved to the msgid/msgstr properties. To disable the warning that this might not be the expected behavior, include { disableSelectWarning: true } in the formatOptions.

  • Source/development languages with more than two plurals could experience difficulties when no custom IDs are used, as gettext cannot have more than two plurals cases identifying an item (msgid and msgid_plural).

  • Gettext doesn’t support plurals for negative and fractional numbers even though some languages have special rules for these cases.


Simple JSON file where each key is message ID and value is translation. The JSON is flat and there’s no reason to use nested keys. The usual motivation behind nested JSON is to save filespace, but this file format is used offline only.

The drawback of this format is that all metadata about message are lost. That includes default message, origin of message and any message flags (obsolete, fuzzy, etc).

   "messageId": "translation"

Lingui (raw)

This file format simply outputs all internal data in JSON format. It’s the original file format used by LinguiJS library before support for other catalog formats were added. It might be useful for tools build on top of Lingui CLI which needs to further process catalog data.

   "messageId": {
      "translation": "Translated message",
      "defaults": "Default message",
      "description": "Comment for translators",
      "origin": [["src/App.js", 3]]
   "obsoleteId": {
      "translation": "Obsolete message",
      "origin": [["src/App.js", 3]],
      "obsolete": true