docs: update GetText doc (#7618)

* Better documentation

* Fix doc

* Fix doc about en.po

* Update labeler.yml

Co-authored-by: Stéphane Gigandet <stephane@openfoodfacts.org>
Co-authored-by: Pierre Slamich <pierre@openfoodfacts.org>

.github/labeler.yml

@@ -176,6 +176,7 @@ footer:
 documentation:
 - docs/**/*
 - scripts/generate_perl_html_doc_from_pod.pl
+- po/README.md
 
 docker:
 - docker/**/*

po/README.md

@@ -1,3 +1,25 @@
 ### Translations
-You can translate Open Food Facts at https://translate.openfoodfacts.org. Please do not translate directly in the code, or your translations will be overwritten.
-If you have added or modified a string in the source file (the file with the .pot extension), you should update the crowdin-trigger branch from main/develop. This will trigger import within Crowdin, and 1 hour later, it will create a new pull request with added or modified strings in all language files, plus all added or modified translations.
+You can translate Open Food Facts at https://translate.openfoodfacts.org thanks to the Crowdin platform. Please do not translate directly in the code, or your translations will be overwritten.
+
+If you want to add a translatable string in the code:
+* modify the file with the `.pot` extension related to your context. Eg.
+  ```pot
+  msgctxt "name_of_your_string"
+  msgid "Hello world!"
+  msgstr ""
+  ```
+* modify the file `en.po` related to your context, to be sure at least an english string will be displayed before Crowdin send back all `.po` files (which can take several days). It should be the same code as the one from `.pot` + the english transaltion (see `msgstr` field). Eg.
+  ```po
+  msgctxt "name_of_your_string"
+  msgid "Hello world!"
+  msgstr "Hello world!"
+  ```
+* use your new string with the `lang()` function. Eg. in a template file: `[% lang('name_of_your_string') %]`
+* if you want to test your modifications in your dev environnement:
+  * modify the `fr.po` files if you want to test in French; be aware it's just for test: these files will be overwritten by Crowdin. Eg. in French:
+    ```pot
+    msgctxt "name_of_your_string"
+    msgid "Hello world!"
+    msgstr "Salut le monde!"
+    ```
+  * don't forget to `make build_lang` to let docker take your modifications into account