As a news poster, I want an option to select the spoken language at the event, so that I can communicate this more easily

### 📝 Description

In the *Kommunikationspolicy*, *§5 PR av evenemang* it is defined that:

> I evenemangets beskrivning, oavsett forum för marknadsföring, bör det vara definierat om språket som kommer att talas under evenemanget är svenska eller engelska.

This does not really happen currently, so to make it easier there could be an explicit option that allows you to select the spoken language at the event. This is then shown in the news feed.

### ✅ Acceptance Criteria

- [ ] There is an option when creating a news post to select if the spoken language at the event is Swedish or English. This option is only mandatory if an event is linked to the news post, since some news posts are not related to any event.
- [ ] The spoken language is shown as an icon on each news post in the feed.
- [ ] The spoken language is also available as text, either as a tooltip on hover or displayed near the icon. Such as "Swedish will be the spoken language at this event"

### 💡 Additional context

Examples of how it could look

Start by locating the news-post creation flow and the news-feed components. Trace how event-linked post data is stored and rendered, then add the Swedish/English selection and its required validation for linked events. Done means the language appears as an icon with accessible text in the feed and works for both event-linked and standalone news posts.

Written by the indexing model from the issue text.