Add multilingual support for exercise descriptions and tips to be displayed in the exercise implementation

[AutoPING-TUI]

Implements and closes #2743

To improve maintenance of exercises in multilingual teaching scenarios it would be great to provide exercises with exercise descriptions in several languages rather than working with translated copies of a task. CodeOcean has already multilingual support. However, it is restricted to its GUI. An promising approach can be Moodle's Multilang Content Filter: Attributes (lang="XX" and class="multilang") are used to indicate the multi-language nature and to specify the exact language of a content element. CSS is then used to control the visibility on the page.

When editing the exercise description the instructor had to use the suggested attributes as shown in the examples below:

Example 1 (separated paragraphs):

Example 2 (Inline separation of multilingual text):

<p><span class="multilang" lang="de">Implementieren Sie eine Funktion, die für einen gegebenen Parameter entscheidet, ob er gerade oder ungerade ist.</span> <span class="multilang" lang="en">Implement a function that decides for a given parameter whether it is even or odd.</span></p>

German:

English:

[MrSerth]

Thanks for adressing all my comments, I really appreciate it! Otherwise, I just made another minor suggestion.

💁‍♂️ We should squash some commits during merge to get a clean, linear history without too many small amendments caused by the review. Please, do not just "rebase" (nor "merge").