Translating software using GNU gettext#

GNU gettext is one of the most widely used tool for internationalization of free software. It provides a simple yet flexible way to localize the software. It has great support for plurals, it can add further context to the translated string and there are quite a lot of tools built around it. Of course it has great support in Weblate (see GNU gettext file format description).

Note

If you are about to use it in proprietary software, please consult licensing first, it might not be suitable for you.

GNU gettext can be used from a variety of languages (C, Python, PHP, Ruby, JavaScript and many more) and usually the UI frameworks already come with some support for it. The standard usage is through the gettext() function call, which is often aliased to _() to make the code simpler and easier to read.

Additionally it provides pgettext() call to provide additional context to translators and ngettext() which can handle plural types as defined for target language.

As a widely spread tool, it has many wrappers which make its usage really simple, instead of manual invoking of gettext described below, you might want to try one of them, for example intltool.

Workflow overview#

The GNU gettext uses several files to manage the localization:

PACKAGE.pot contains strings extracted from your source code, typically using xgettext or some high level wrappers such as intltool.
LANGUAGE.po contains strings with a translation to single language. It has to be updated by msgmerge once the PACKAGE.pot is updated. You can create new language files using msginit or within Weblate.
LANGUAGE.mo contains binary representation of LANGUAGE.po and is used at application runtime. Typically it is not kept under version control, but generated at compilation time using msgfmt. In case you want to have it in the version control, you can generate it in Weblate using Generate MO files add-on.

Overall the GNU gettext workflow looks like this:

Sample program#

The simple program in C using gettext might look like following:

#include <libintl.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int count = 1;
    setlocale(LC_ALL, "");
    bindtextdomain("hello", "/usr/share/locale");
    textdomain("hello");
    printf(
        ngettext(
            "Orangutan has %d banana.\n",
            "Orangutan has %d bananas.\n",
            count
        ),
        count
    );
    printf("%s\n", gettext("Thank you for using Weblate."));
    exit(0);
}

Extracting translatable strings#

Once you have code using the gettext calls, you can use xgettext to extract messages from it and store them into a .pot:

$ xgettext main.c -o po/hello.pot

Note

There are alternative programs to extract strings from the code, for example pybabel.

This creates a template file, which you can use for starting new translations (using msginit) or updating existing ones after code change (you would use msgmerge for that). The resulting file is simply a structured text file:

# SOME DESCRIPTIVE TITLE.
# Copyright (C) YEAR THE PACKAGE'S COPYRIGHT HOLDER
# This file is distributed under the same license as the PACKAGE package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: PACKAGE VERSION\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2015-10-23 11:02+0200\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=CHARSET\n"
"Content-Transfer-Encoding: 8bit\n"
"Plural-Forms: nplurals=INTEGER; plural=EXPRESSION;\n"

#: main.c:14
#, c-format
msgid "Orangutan has %d banana.\n"
msgid_plural "Orangutan has %d bananas.\n"
msgstr[0] ""
msgstr[1] ""

#: main.c:20
msgid "Thank you for using Weblate."
msgstr ""

Each msgid line defines a string to translate, the special empty string in the beginning is the file header containing metadata about the translation.

Starting new translation#

With the template in place, we can start our first translation:

$ msginit -i po/hello.pot -l cs --no-translator -o po/cs.po
Created cs.po.

The just created cs.po already has some information filled in. Most importantly it got the proper plural forms definition for chosen language and you can see number of plurals have changed according to that:

# Czech translations for PACKAGE package.
# Copyright (C) 2015 THE PACKAGE'S COPYRIGHT HOLDER
# This file is distributed under the same license as the PACKAGE package.
# Automatically generated, 2015.
#
msgid ""
msgstr ""
"Project-Id-Version: PACKAGE VERSION\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2015-10-23 11:02+0200\n"
"PO-Revision-Date: 2015-10-23 11:02+0200\n"
"Last-Translator: Automatically generated\n"
"Language-Team: none\n"
"Language: cs\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=ASCII\n"
"Content-Transfer-Encoding: 8bit\n"
"Plural-Forms: nplurals=3; plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;\n"

#: main.c:14
#, c-format
msgid "Orangutan has %d banana.\n"
msgid_plural "Orangutan has %d bananas.\n"
msgstr[0] ""
msgstr[1] ""
msgstr[2] ""

#: main.c:20
msgid "Thank you for using Weblate."
msgstr ""

This file is compiled into an optimized binary form, the .mo file used by the GNU gettext functions at runtime.

Updating strings#

Once you add more strings or change some strings in your program, you execute again xgettext which regenerates the template file:

$ xgettext main.c -o po/hello.pot

Then you can update individual translation files to match newly created templates (this includes reordering the strings to match new template):

$ msgmerge --previous --update po/cs.po po/hello.pot

Importing to Weblate#

To import such translation into Weblate, all you need to define are the following fields when creating component (see Component configuration for detailed description of the fields):

Field	Value
Source code repository	URL of the VCS repository with your project
File mask	`po/*.po`
Template for new translations	`po/hello.pot`
File format	Choose gettext PO file
New language	Choose Create new language file

And that’s it, you’re now ready to start translating your software!