Documentation translation (#199)

Documentation translation using weblate.

* Use po4a to generate .po files from the english markdowns, then to generate translated files
* Some pages can be marked as «english only» (for technical documentation for example)
* New Hugo shortcode to use application strings in documentation (for example for settings names)
* The code of conduct is no more translated, but there is a link to official Contributor Covenant translations
* Adding all plugin's supported languages

Related to https://github.com/JohnXLivingston/peertube-plugin-livechat/issues/117

support/documentation/content/en/contributing/translate/_index.md

@@ -0,0 +1,112 @@
+---
+title: "Translate"
+description: "Translate the plugin"
+weight: 20
+chapter: false
+---
+
+You can contribute to this plugin translation.
+Translations are handled using the [Weblate](https://weblate.org/) software,
+using [Framasoft Weblate instance](https://weblate.framasoft.org/).
+
+{{% notice warning %}}
+Never modify directly files in the `languages` folder, this could lead to conflicts.
+{{% /notice %}}
+
+## How to
+
+* Create an account: https://weblate.framasoft.org/accounts/register/
+* Validate your email and follow the link sent
+* Create your password and setup your account
+* Go to the plugin project page: https://weblate.framasoft.org/projects/peertube-livechat/peertube-plugin-livechat/
+* Choose the locale you want to translate
+* Just translate missing sentences, or correct the ones that seems incorrect to you.
+
+{{% notice warning %}}
+There might be some «very technical» strings. If you are not 100% sure of
+the meaning, or of your translation, you better not translate it,
+so it will display in english.
+{{% /notice %}}
+
+## Adding a new locale
+
+If you think there is a missing locale, please check first if it is handled in Peertube.
+If so, you can [open an issue](https://github.com/JohnXLivingston/peertube-plugin-livechat/issues) to ask for it.
+
+## Adding new strings / use translations in the code
+
+If you are working on new features, and need new strings, you can create them directly in Weblate.
+The english version is mandatory. Start with it.
+
+Each string is linked to a key (for example `use_chat`).
+Choose an explicit key in english, lower case.
+
+If you have to test new strings without waiting for a Weblate merge, you can modify `languages/*.yml` files,
+but avoid to commit these changes (to minimize conflict risks).
+
+### Use translations in front-end code
+
+Before using a string in front-end, you need to declare a new constant in `client/@types/global.d.ts`.
+The constant name must:
+
+* start with the prefix "LOC_"
+* use the string key, upper cased
+* you just have to declare its type, not its value
+
+For example, to use "use_chat", you have to declare:
+
+```typescript
+declare const LOC_USE_CHAT: string
+```
+
+The `build-client.js` script will read the `client/@types/global.d.ts`,
+search for such constants, and load their values from the languages files.
+
+Now, you can simply call `peertubeHelpers.translate(LOC_USE_CHAT)` in your code.
+
+### Use translations in back-end code
+
+In theory, the only parts of the backend code where you need localization is the
+settings declaration and standardized data (ActivityPub, RSS, ...).
+Here we need to get english strings from the translation key.
+
+Note: you should never need another language translation from backend code.
+Localization must be done on front-end.
+
+There is a `lib/loc.ts` module providing a `loc()` function.
+Just pass it the key to have the english string: `loc('diagnostic')`'.
+
+## Documentation translation
+
+The documentation translation is done using the corresponding Weblate component.
+
+There is a specific "Hugo shortcodes" you can use to display an application string.
+Saying you want to display the name of the button "open_chat_new_window", you can use this in the documentation markdown file:
+
+```hugo
+{{%/* livechat_label open_chat_new_window */%}}
+```
+
+You can also prevent a whole page from being translating by adding `livechatnotranslation: true` in the Yaml Font Matter section:
+
+```yaml
+---
+title: "Third party"
+description: "Displaying the livechat with 3rd party software."
+weight: 20
+chapter: false
+livechatnotranslation: true
+---
+```
+
+Never translate a string in the `livechat.en.pot` file, it would be ignored.
+Instead, edit directly the markdown files.
+
+If a string contains a link, you can change it for the correct link in the translated language.
+For example, for a link to this documentation, you can add the language code in the url.
+
+Some strings are block of code.
+Don't translate code.
+But you can translate comments, or parameters if relevant.
+
+If you are not sure, just don't translate, and ask what to do.