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CLIENT-SPECIFICATION: clarify fallback to english #4101

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Jul 21, 2020
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CLIENT-SPECIFICATION: clarify fallback to english #4101

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01cf9d8
CLIENT-SPECIFICATION: clarify fallback to english
sbrl Jun 11, 2020
14508cf
Update CLIENT-SPECIFICATION.md
sbrl Jun 11, 2020
e69a64e
Update CLIENT-SPECIFICATION.md
zlatanvasovic Jun 19, 2020
c22a3de
english -> English
zlatanvasovic Jun 19, 2020
d19c315
client-spec: make english fallback a MUST
sbrl Jun 21, 2020
ebeb655
client-spec: add example table.
sbrl Jun 23, 2020
2019aec
client-spec: update language wording fo initial paragraph
sbrl Jun 27, 2020
b7c93cf
Update CLIENT-SPECIFICATION.md
sbrl Jun 27, 2020
e83929f
Update CLIENT-SPECIFICATION.md
sbrl Jun 27, 2020
ec855f9
Update CLIENT-SPECIFICATION.md
sbrl Jun 27, 2020
99a3402
client-spec: fix table & update changelog
sbrl Jun 27, 2020
35fc27b
Update CLIENT-SPECIFICATION.md
mebeim Jun 27, 2020
ea538a9
Client spec: clarify usage of language env vars.
mebeim Jul 13, 2020
ccdcdb4
Update CLIENT-SPECIFICATION.md
mebeim Jul 18, 2020
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42 changes: 25 additions & 17 deletions CLIENT-SPECIFICATION.md
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Expand Up @@ -173,29 +173,37 @@ If multiple versions of a page were found for different platforms, then a client

## Language

Pages can be written in multiple languages. If a client has access to environment variables, several standard ones exist to specify the language in which a client should operate. If not, then clients MUST make reasonable assumptions based on the information provided by the environment in which they operate (e.g. consulting `navigator.languages` in a browser, etc.). If possible, it is RECOMMENDED to also make language configurable, as to not only rely on the environment. Clients SHOULD therefore offer options to configure or override the language using configuration files or command line options (like `-L, --language` as suggested in the [arguments section](#arguments) above).
Pages can be written in multiple languages. If a client has access to environment variables, it MUST use them derive the preferred user language as described in the next paragraphs. If not, then clients MUST make reasonable assumptions based on the information provided by the environment in which they operate (e.g. consulting `navigator.languages` in a browser, etc.).

The [`LANG` environment variable](https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html), if present, MUST be used to determine the language of pages to display.
The [`LANG` environment variable](https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html) specifies the user preferred locale (in the form `ll[_CC][.encoding]`). The [`LANGUAGE` environment variable](https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html) specifies a priority list of locales (in the form `l1:l2:...`) that can be used if the locale defined by `LANG` is not available. Both `LANG` and `LANGUAGE` may contain the values `C` or `POSIX`, which should be ignored.

The [`LANGUAGE` environment variable](https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html) specifies a priority list of languages that a user wishes to read in. The `LANG` environment variable MUST be present for `LANGUAGE` to be used, and then `LANGUAGE` takes precendence over `LANG`. If `LANG` is set to a language not in `LANGUAGE`, then `LANG` should be appended to the end of the priority list. If a page is not available in the user's preferred language, then a client MUST respect the user's priority list defined in the `LANGUAGE` variable, and MAY choose to notify the user that a page in their chosen language couldn't be found (perhaps along with a link to the [translations section of the contributing guide](https://github.com/tldr-pages/tldr/blob/master/CONTRIBUTING.md#translations)).
In order to determine the display language, a client MUST:

Regardless of the language selected through the above environment variables, clients MUST always attempt to fallback to English if the page does not exist in the requested languages. In this case clients SHOULD tell the user that the page does not exist in their requested language if it was not English. If the client supports a command-line argument for language, the client MUST only attempt to show the page in that language (clients MAY notify the user that a page is available in other languages if present).
1. Check the value of `LANG`.
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2. Extract the priority list from `LANGUAGE`. If not set, start with an empty priority list.
3. Append the value of `LANG` to the priority list.
4. Follow the priority list in order and use the first available language.
5. Fall back to English if none of the languages are available.

LANGUAGE | LANG | Result
------------|-------|----------
`it:cz:de` | `cz` | `it`, `cz`, `de`, `en`
`it:de:fr` | `cz` | `it`, `de`, `fr`, `cz`, `en`
-- | `it` | `it`, `en`
`it:cz` | -- | `en`
-- | -- | `en`
Examples:

Note: `LANG` or `LANGUAGE` may contain the values `C` or `POSIX`, which should be ignored.
LANG | LANGUAGE | Result
-------|-----------|-----------------------------
`cz` |`it:cz:de` | `it`, `cz`, `de`, `en`
`cz` |`it:de:fr` | `it`, `de`, `fr`, `cz`, `en`
`it` |unset | `it`, `en`
unset |`it:cz` | `en`
unset |unset | `en`

Regardless of the language determined through the environment, clients MUST always attempt to fallback to English if the page does not exist in the user preferred language. Clients MAY notify the user when a page in their preferred language cannot be found (optionally including a link to the [translations section of the contributing guide](https://github.com/tldr-pages/tldr/blob/master/CONTRIBUTING.md#translations)).

It is also RECOMMENDED to make the language configurable, as to not only rely on the environment. Clients SHOULD offer options to configure or override the language using configuration files or even command line options (like `-L, --language` as suggested in the [arguments section](#arguments) above). If such a command-line option is specified, a client must strictly adhere to its value, and MUST NOT show pages in a different language, failing with an appropriate error message instead.

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The [`LC_MESSAGES` environment variable](https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html) MAY be present. If the client itself is localized and this environment variable is present, it MUST use its value in order to determine the language in which interface text is shown (separately from the language used for pages). In absence of `LC_MESSAGES`, then `LANG` and `LANGUAGE` MUST be used for this purpose instead.

**Note that** it is highly RECOMMENDED to give precedence to the platform first, and then the language. In other words, look for a platform under each language, before falling back to the next preferred language. This ensures a meaningful and correct page resolution.
**Note that** for page lookup it is highly RECOMMENDED to give precedence to the platform over the language. In other words, look for a platform under each language, before checking the next preferred language. This ensures a meaningful and correct page resolution.

Here's an example of how the lookup should be done on `linux` having set `LANGUAGE="it:fr:en"`:
Here's an example of how the lookup should be done on `linux` having set `LANG=it` and `LANGUAGE="it:fr:en"`:

1. pages.it/linux/some-page.md -> does not exist
2. pages.fr/linux/some-page.md -> does not exist
Expand All @@ -206,15 +214,15 @@ Here's an example of how the lookup should be done on `linux` having set `LANGUA

## Caching

If appropriate, it is RECOMMENDED that clients implement a cache of pages. If implemented, clients MUST download the archive either from **[http://tldr.sh/assets/tldr.zip](http://tldr.sh/assets/tldr.zip)** or [https://raw.githubusercontent.com/tldr-pages/tldr-pages.github.io/master/assets/tldr.zip](https://raw.githubusercontent.com/tldr-pages/tldr-pages.github.io/master/assets/tldr.zip) (which is pointed by the first link).
If appropriate, it is RECOMMENDED that clients implement a cache of pages. If implemented, clients MUST download the archive either from **[http://tldr.sh/assets/tldr.zip](http://tldr.sh/assets/tldr.zip)** or [https://raw.githubusercontent.com/tldr-pages/tldr-pages.github.io/master/assets/tldr.zip](https://raw.githubusercontent.com/tldr-pages/tldr-pages.github.io/master/assets/tldr.zip) (which is pointed to by the first link).

Caching SHOULD be done according to the user's language configuration (if any), as to not waste unneeded space for unneeded languages. Additionally, clients MAY automatically update the cache on a regular basis.
Caching SHOULD be done according to the user's language configuration (if any), as to not waste unneeded space for unused languages. Additionally, clients MAY automatically update the cache on a regular basis.


## Changelog

- [v1.3, June 11th 2020](https://github.com/tldr-pages/tldr/blob/master/CLIENT-SPECIFICATION.md) (#4101)
- Clarified fallback to English in the language resolution algorithm.
- Clarified fallback to English in the language resolution algorithm.
- Update `LANG` and `LANGUAGE` environment variable to conform to the GNU spec.

- [v1.2, July 3rd 2019](https://github.com/tldr-pages/tldr/blob/524d44eb13ff6c0ff70089bd152b075418fc71b2/CLIENT-SPECIFICATION.md) (#3168)
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