docs(i18n): make translation guide more explicit (#18254)

docs/docs/Contributing/translations.mdx

@@ -7,14 +7,19 @@ version: 1
 
 ## Translating
 
-We use [Babel](http://babel.pocoo.org/en/latest/) to translate Superset.
-In Python files, we import the magic `_` function using:
+We use [Flask-Babel](https://flask-babel.tkte.ch/) to translate Superset.
+In Python files, we use the following
+[translation functions](https://flask-babel.tkte.ch/#using-translations) from
+`Flask-Babel`:
+- `gettext` and `lazy_gettext` (usually aliased to `_`): for translating singular
+  strings.
+- `ngettext`: for translating strings that might become plural.
 
 ```python
 from flask_babel import lazy_gettext as _
 ```
 
-then wrap our translatable strings with it, e.g. `_('Translate me')`.
+then wrap the translatable strings with it, e.g. `_('Translate me')`.
 During extraction, string literals passed to `_` will be added to the
 generated `.po` file for each language for later translation.
 
@@ -43,29 +48,80 @@ LANGUAGES = {
 }
 ```
 
+### Creating a new language dictionary
+
+First check if the language code for your target language already exists. Check if the
+[two letter ISO 639-1 code](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes)
+for your target language already exists in the `superset/translations` directory:
+
+```bash
+ls superset/translations | grep -E "^[a-z]{2}\/"
+```
+
+If your language already has a pre-existing translation, skip to the next section
+
+The following languages are already supported by Flask AppBuilder, and will make it
+easier to translate the application to your target language:
+[Flask AppBuilder i18n documentation](https://flask-appbuilder.readthedocs.io/en/latest/i18n.html)
+
+To create a dictionary for a new language, first make sure the necessary dependencies are installed:
+```bash
+pip install -r superset/translations/requirements.txt
+```
+
+Then run the following, where `LANGUAGE_CODE` is replaced with the language code for your target
+language:
+
+```bash
+pybabel init -i superset/translations/messages.pot -d superset/translations -l LANGUAGE_CODE
+```
+
+For instance, to add a translation for Finnish (language code `fi`), run the following:
+
+```bash
+pybabel init -i superset/translations/messages.pot -d superset/translations -l fi
+```
+
 ### Extracting new strings for translation
 
+This step needs to be done every time application strings change. This happens fairly
+frequently, so if you want to ensure that your translation has good coverage, this
+step needs to be run fairly frequently and the updated strings merged to the upstream
+codebase via PRs. To update the template file `superset/translations/messages.pot`
+with current application strings, run the following command:
+
 ```bash
 pybabel extract -F superset/translations/babel.cfg -o superset/translations/messages.pot -k _ -k __ -k t -k tn -k tct .
 ```
 
-This will update the template file `superset/translations/messages.pot` with current application strings. Do not forget to update
-this file with the appropriate license information.
+Do not forget to update this file with the appropriate license information.
 
 ### Updating language files
 
+Run the following command to update the language files with the new extracted strings.
+
 ```bash
  pybabel update -i superset/translations/messages.pot -d superset/translations --ignore-obsolete
 ```
 
-This will update language files with the new extracted strings.
-
 You can then translate the strings gathered in files located under
-`superset/translation`, where there's one per language. You can use [Poedit](https://poedit.net/features)
+`superset/translation`, where there's one folder per language. You can use [Poedit](https://poedit.net/features)
 to translate the `po` file more conveniently.
 There are some [tutorials in the wiki](https://wiki.lxde.org/en/Translate_*.po_files_with_Poedit).
 
-In the case of JS translation, we need to convert the PO file into a JSON file, and we need the global download of the npm package po2json.
+To perform the translation on MacOS, you can install `poedit` via Homebrew:
+
+```bash
+brew install poedit
+```
+
+After this, just start the `poedit` application and open the `messages.po` file. In the
+case of the Finnish translation, this would be `superset/translations/fi/LC_MESSAGES/messages.po`.
+
+### Applying translations
+
+To make the translations available on the frontend, we need to convert the PO file into
+a JSON file. To do this, we need to globally install the npm package `po2json`.
 
 ```bash
 npm install -g po2json
@@ -84,20 +140,9 @@ the executable path (e.g. `/usr/local/bin/po2json` instead of `po2json`).
 If you get a lot of `[null,***]` in `messages.json`, just delete all the `null,`.
 For example, `"year":["年"]` is correct while `"year":[null,"年"]`is incorrect.
 
-For the translations to take effect we need to compile translation catalogs into binary MO files.
+Finally, for the translations to take effect we need to compile translation catalogs into
+binary MO files.
 
 ```bash
 pybabel compile -d superset/translations
 ```
-
-### Creating a new language dictionary
-
-To create a dictionary for a new language, run the following, where `LANGUAGE_CODE` is replaced with
-the language code for your target language, e.g. `es` (see [Flask AppBuilder i18n documentation](https://flask-appbuilder.readthedocs.io/en/latest/i18n.html) for more details):
-
-```bash
-pip install -r superset/translations/requirements.txt
-pybabel init -i superset/translations/messages.pot -d superset/translations -l LANGUAGE_CODE
-```
-
-Then, [extract strings for the new language](#extracting-new-strings-for-translation).