How to enable "multi-lingual" insertion of data for administrators, while having a separate main site language

Our website allows certain "administrators" to add events and manage those events. Then "users" can enroll said events.

One of the features that is requested is multi-lingual support "on event" level. Which means that administrators can translate their own events for users. Any user (and also administrator) could select the main site language, which is used to display both the event in said language (if provided) as well as main interface.

Now the problem is, how would I allow/make it clear for administrators to insert translations for multiple languages. -- Inserting a translation shouldn't require first changing the main language right?

I could add a "per-field" box to select language, but this would mean that at multiple pages you have to keep changing the language, and the box isn't clear when there are multiple fields on the same page.

As a reference below is one page an administrator might edit:

Both the description as well as the "gebruiksvoorwaarden" (terms) would be editable. -- And while the main language selected is "dutch", the administrator should(?) be able to add translations also in other languages like french and english.

Where would I put the interface icon for this? Especially considering such fields might also exist on other "tabs" - all tabs without a cog symbol are visible to the end user, and hence the dynamic fields on all those tabs need to be translated as well.

Tot the right of "test-event" bar? But how would I make it clear which field is the interface language, and what is the language you are currently editing as?