With Windows 10, version 1803, we started to deliver language translation improvements via Local Experience Packs, also referred to as LXPs. Local Experience Packs are AppX packages delivered via the...
Updated May 02, 2019
Version 3.0
Blog Post
Valter,
As Milen recommended, in 1809 you will have to add the language to the preferred list as it will not show up in the Windows display language drop down. We are working on making the pre-installed languages show up in the Windows display language dropdown so expect to see that in future releases.
For now, please click on Install after you see offline error message. That should add the language to user preferred language list as well as make it available in the Display Language dropdown.
For SKU languages, you do not need to get the LXP or connect to the Microsoft Store. All 3 of the languages that you are installing - en-gb; es-es; el-gr - are SKU languages. You can read more about LXPs here: Local Experience Packs: What are they and when should you use them?