Add language configuration and Spanish translation

This commits configures and sets up flask-babel and adds Spanish po
file. Flask-babel is already a dependency so this commits no needs to
add it.

Users can edit ogcp.json to choose between English or Spanish. After
changing the language in the configuration file, users must restart ogcp
service.

Instructions to generate translation files, inspired by
https://flask-babel.tkte.ch/:

1. Download ogcp source code.

   git clone git://git.soleta.eu/ogCP /path/to/ogcp

2. Create a python virtual environment.

   python3 -m venv /path/to/new/venv

3. Activate the virtual environment.

   source /path/to/new/venv/bin/activate

4. Install dependencies to the virtual environment.

   cd /path/to/ogcp
   pip3 install -r requirements.txt

5. Extract ogcp strings.

   cd /path/to/ogcp/ogcp
   pybabel extract -F babel.cfg -k _l -o messages.pot .

   NOTE: "-k _l" is needed to extract strings which use lazy
   translations.

6. Create or update translation files.

   a. Create translation file. This example creates a french translation
   file.

      pybabel init -i messages.pot -d translations -l fr

   b. Update translation files. You need to do this when ogcp strings
   change and want to update current supported languages.

      pybabel update -i messages.pot -d translations

7. Edit the translations .po file as needed. Example route:
   /path/to/ogcp/ogcp/translations/fr/LC_MESSAGES/messages.po.

8. Compile translations.

   pybabel compile -d translations

9. Launch ogcp and check the translations.

   cd /path/to/ogcp
   pip3 install -r requirements.txt

10. Commit and push .po files with translation changes.

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