# Localization workflow

Since it is impractical to request of a programmer to know by heart a magic
key referring to a specific message, it stands to reason that the localization
system should provide an ergonomic way to refer to these keys in a symbolic
manner (remember the abstract example for `MSG_Hello_World` above.

Following the best practices for 18n and l10n, the source strings live in an
XML file (here, named `strings.xml`), explained below. An
[example of a `strings.xml`](/src/intl/example/strings.xml) file is shown below.
The goal of this file is to declare all externalized strings that our program
uses, and give them a locally unique `name`.  The strings will be used as a
basis for translation, and `name`s will be used as a basis for the symbolic
```xml
<!-- comment -->
<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">
  <!-- comment -->
  <string
    name="STRING_NAME"
      >text_string</string>
  <string
    name="STRING_NAME_2"
      >text_string_2</string>
  <string
    name="STRING_NAME_3"
      >string with
an intervening newline</string>
</resources>
```

The file `strings.xml` goes through a series of transformations in which
language-specific varieties of the same file are produced by translators.  The
input-output behavior of the translation process is: `strings.xml` file goes
in, with strings written in some source (human) language, and multiple flavors
of `strings.xml` come out, each translated in a particular single language.
The entire translation process can be quite involved, in a large organization
it can involve farming tasks out to translators who may live around the world,
and scores of dedicated translation tooling. but the precise mechanics of the
box does not matter too much to us as consumers as long as the input-output
behavior of the process is upheld, and we're generally aware that the
translation could take a while.  The resulting files are converted into a
machine-readable form, and shipped alongside a Fuchsia program within the same
[Fuchsia package](/docs/glossary/README.md#package).
An important feature of Fuchsia packages is that they are inherently not an
archive, but rather a manifest that points to files by their content hash.  So
multiple programs can share the same files, and languages closely related
("en-US", "en-GB") can potentially share message disk space. The following
diagram shows a compact overview of the lifecycle of strings.

![The above image shows the localization flow. Since XML files are annotated they are not directly suitable for machine translation, so we convert to JSON files, for which we can reuse available libraries to load them, and construct a map from a key to message string.  These strings can then be used as format strings in `MessageFormat`.](images/localization-workflow.png "The above image shows the localization flow. Since XML files are annotated they are not directly suitable for machine translation, so we convert to JSON files, for which we can reuse available libraries to load them, and construct a map from a key to message string.  These strings can then be used as format strings in `MessageFormat`.")

### `strings.xml`

We are reusing the Android string resources XML format to represent localizable
strings. Since we will be adding nothing to the strings.xml format, the full
discussion of the features is delegated to the [string resources
page](https://developer.android.com/guide/topics/resources/string-resource).

While all that XML in the above diagram makes this discussion look like it just
emerged from some wormhole connected straight to the 1990s, XML is actually a
very good fit to describe annotated text. strings.xml is a format that has been
time-tested in Android so we know it will be adequate, and developers are
familiar with it.

For example, a string resource can be declared with annotations interleaved
into the source text.

```xml
<!-- … -->
<string name="title"
   >Best practices for <annotation font="title_emphasis">text</annotation> look like so</string>
<!-- … -->
```
Above: An example interleaving of translation text and annotation._

It is possible to interleave text that should be _protected from translation_,
[like so](https://developer.android.com/guide/topics/resources/localization):


```xml
<string name="countdown">
  <xliff:g id="time" example="5 days"
    >{1}</xliff:g> until holiday</string>
```

Above:: An example of an interleaving of a fenced-off parameter, annotated with
an example value and guarded with a tag that is not part of the string
resources data schema._

We can also define our own additions to the data schema if we so need, and
interleave that data schema _transparently_ in an existing schema.

There are some necessary constraints on the contents of the file above:

*   Every `name` attribute in the file must be unique.
*   Name identifiers may contain uppercase and lowercase ASCII letters, digits and underscores, but may not start with a number.  So for example,` _H_e_L_L_o_wo_1_rld` is allowed, but `0cool` is not.
*   No two `name`-`message` combinations in the file may repeat.

For the time being there are no provisions for having multiple strings files in
a project.

### Message identifiers

The message identifiers (the "magical" numeric constants for each message) are
generated based on the contents of the `strings.xml` file.  Every string
message gets a unique identifier, which is computed based on the one-way hash
on `name` and the contents of the message itself.  This identifier assignment
ensures that it is vanishingly unlikely for two different messages to
accidentally have the same resulting identifier.

The generation of these messages is automated by
[GN](https://gn.googlesource.com/gn/) build rules in Fuchsia, but is ultimately
performed by a program called
[strings_to_fidl](/src/intl/strings_to_fidl/README.md).
This program generates FIDL intermediate representation for the message IDs,
and the regular FIDL toolchain is used to produce language-specific versions of
that info.  As an example, the C++ flavor would be a header file with the
following content:


```cpp
namespace fuchsia {
namespace intl {

namespace l10n {
enum class MessageIds : uint64_t {
  STRING_NAME = 42u,
  STRING_NAME_2 = 43u,
  STRING_NAME_3 = 44u,
};

}  // namespace l10n
}  // namespace intl
}  // namespace fuchsia
```

The precise values assigned to each particular enum value in the example above
are not relevant.  The generation method is also not relevant at this time,
since all identifiers are generated at compile time and there is no opportunity
for version skew.  We may for now safely assume that an identical name-content
combination will _always_ have the same message ID assigned.

It is fairly easy to include the resulting file into a C++ program.  A minimal
example is given below, but refer to the fully worked-out example for the
precise details of the wire-up.  The library parameter `fuchsia.intl.l10n` is
provided directly by the author as a flag to `strings_to_fidl`; or if the
appropriate GN template is used, as a parameter to the GN template.

```cpp
#include <iostream>

// This header file has been generated from the strings library fuchsia.intl.l10n.
#include "fuchsia/intl/l10n/cpp/fidl.h"

// Each library name segment between dots gets its own nested namespace in
// the generated C++ code.
using fuchsia::intl::l10n::MessageIds;

int main() {
  std::cout << "Constant: " << static_cast<uint64_t>(MessageIds::STRING_NAME) << std::endl;
  return 0;
}
```

### \*.json

The FIDL and C++ code generation makes the message IDs available to the program
authors.  On the packaging side, we also must provide the localized asset for
each language we support.  At present the encoding for this information is
JSON.  This was done for expedience, but a number of improvements can be
made on that decision to improve performance and security.

Generating this information is delegated to the program named
[strings_to_json](/src/intl/strings_to_json/README.md),
which merges the original `strings.xml` with a language specific
file (for example, a French translation lives in `strings_fr.xml`).
Again, for builds driven by GN, the invocation of `strings_to_json`
is encapsulated in a build rule.

Example contents of a generated JSON file are given below.


```json
{
  "locale_id": "fr",
  "source_locale_id": "en-US",
  "num_messages": 3,
  "messages": {
    "42": "le string",
    "43": "le string 2",
    "44": "le string\nwith intervening newline"
  }
}
```


The JSON format has the following fields currently defined.  In case the table
below goes out of date, the source of truth for the JSON structure is the
[strings
model](/src/lib/intl/strings/src/json.rs#47).


| **Field** | **Type** | **Description** |
|-----------|----------|-----------------|
`locale_id` | Locale ID (string) | The locale for which the messages are translated. |
`source_locale_id` | Locale ID (string) | The locale of the source message file. |
`num_messages` | Positive integer | The number of messages that were present in the *original* `strings.xml`.  This allows us to estimate quickly the quality of the translation by comparing that number of messages with the number of messages that are present in the JSON file. |
`messages` | Map `[u64->string]` | A map from message ID to the appropriate message. |