docs: i18n: update, consolidate in a README file

Author: NathanLovato

i18n/README.md

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+# i18n folder - working with translations
+
+The app offers internationalization support using PO (gettext) files.
+
+It's a popular format among translation professionals, and it has native support in Godot.
+
+We keep the translation files in a separate repository. To contribute translations, please head to the [learn-gdscript-translations](https://github.com/GDQuest/learn-gdscript-translations) repository, where you'll find detailed instructions.
+
+This folder contains the app's included translations (those that are complete or nearly complete) and scripts to manage them.
+
+## How translations work in Godot and the app
+
+The engine automatically translates UI nodes, while we need to wrap string literals in calls to the `tr()` function.
+
+In your typical Godot project, you would preload translation resources through the project settings.
+
+Instead, this app loads translations dynamically because we separate localization resources into several files per supported language. This applies to both POT files (translation templates) and translated PO files.
+
+## Updating translation templates
+
+The gettext translation format comes with two file extensions: POT for translation templates and PO for translations to a specific language. PO files derive from POT files, which allow us to track strings that changed between releases finely.
+
+You generate the template POT files using a Python script included in this folder: extract.py.
+
+To use the script, first create and activate a Python virtual environment. This allows you to install Python libraries this project needs without affecting your whole computer's Python installation.
+
+Activate the environment and install dependencies with these commands (on Unix-like systems):
+
+```sh
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r requirements.txt
+```
+
+After doing that once, you can directly activate the environment:
+
+```sh
+source .venv/bin/activate
+```
+
+Then, run the extraction script:
+
+```sh
+python i18n/extract.py
+```
+
+You need to run it from the project's root directory. You then need to copy the POT files over to the [learn-gdscript-translations](https://github.com/GDQuest/learn-gdscript-translations) repository where we translate the content.
+
+### Adding a new language to the app
+
+To add a new language to the app, you need to:
+
+1. Create a new subdirectory in the [learn-gdscript-translations](https://github.com/GDQuest/learn-gdscript-translations) repository with the two-letter code of your language. For example, `es/` for Spanish, `ru/` for Russian, or `zh/` for Chinese.
+2. For each POT file in the repository's root folder, create a derived PO translation file in your new subdirectory. You can use the program [Poedit](https://poedit.net/) to do so.
+3. In this repository, open the `TranslationManager.gd` script and add your new language code to its `SUPPORTED_LOCALES` constant. The order of languages in `SUPPORTED_LOCALES` defines the order they'll appear in the settings menu.
+
+The app may not support your language's characters. If that is the case, please [open an issue](https://github.com/GDQuest/learn-gdscript/issues) on the repository to add the missing font files and font switching code.
+
+**Important:** Currently, right-to-left languages like Arabic are not supported. The app needs to be ported to Godot 4 first, which will make it easier to add support for right-to-left languages.