[Maps] Build localization framework into GIS app

[rayafratkina]

Describe the feature:
The new GIS app should include localization capabilities to allow us to translate it later.
It should also utilize locale-appropriate map data per #23407

Describe a specific use case for the feature:

cc @elastic/kibana-gis @jinmu03

[jinmu03]

@epixa @azasypkin

[azasypkin]

Hey @elastic/kibana-gis ,

What is exactly the scope of this work? Is it related to the text labels we create inside Kibana GIS app/plugin itself or the ones that you receive from the 3rd-party server? If the former, feel free to start with Guide-level explanation and Technical-level explanation from #17201 and let us know if you have any questions or doubts.

[thomasneirynck]

The scope is two-fold here:

• integration of EMS-localization: This is the piece that we do not have a handle on. It is basically ensuring EMS publishes multi-language layer files, and that this is consumed correctly in the GIS-app. It is blocked by Provide a way to select locale for map layer data #23407.
• integration of Kibana-localization: this is the more conventional effort. Ensuring that our UX (buttons, toasts, ...) are multi-language. For this we can piggy-back directly on the current Kibana work (cf. your link RFC: Kibana Internationalization #17201 is a good start for this)

[thomasneirynck]

After internal discussion, it's unlikely this will make 6.7 and 7.0