Add language selection

[ManuelMoeri]

Add the functionality to switch between languages. This also includes making sure that every string is correctly defined and translated for each language.

Talk with @Robin481 about the design. Maybe create a small mockup to show.

TODO:

• Check the languages.yml files for any mistakes
• Make sure both german and english are fully translated
• Set the design
• Implement the language selector
• Make the language selector automagically detect the available languages and display all those options
• Tests

Ⓜ️Ⓜ️

Workflow

To improve code readability and maintainability, follow these guidelines:

1. Check for Hardcoded Strings:
   Open the file and identify any hardcoded strings or overly complicated translations. Replace them with appropriate translation methods.

2. Model Attributes in Forms:
   Use f.label :attr_name for form labels. Add the translation under activerecord.attributes.{modelname}.{attrname} in the YML file.

3. Model Attributes Outside Forms:
   For model attributes outside forms, use {model_instance}.human_attribute_name(:attr_name) or {ModelClass}.human_attribute_name(:attr_name).
   Example: @person.human_attribute_name(:person_roles) or Person.human_attribute_name(:person_roles).
   Refer to the documentation for more details.

4. Link Text:
   For link texts, check if a link action helper can be used. Refer to the ActionsHelper documentation for guidance.

5. Errors
   https://stackoverflow.com/a/17935975

6. Other Translations:
   For other types of text, consider using lazy lookup translations or standard translation methods as outlined in the Rails I18n guide.

[kcinay055679]

NOTES:

• Outsource trainee level slider in partial? (Potential external ticket)

The Language selector doesnt seem to persist the selected language.
Everything else works