In the newest version of the Launcher only a limited number of languages are listed in the drop-down list from settings. Not clear why this is but my Launcher was set to Íslenska (Icelandic) but after the update the UI had been set to English. When I go to settings and click the Language drop-down list, Íslenska (Icelandic) is not among the languages displayed there. There also seems to be a lot of other languages missing from the list.
MCL-11147
Languages not available in newest version of MCL
Resolved
Invalid
0
The Icelandic translation is not completely finished yet. If you want to help out, you can help with the translation on the Minecraft Launcher Crowdin project. Icelandic is currently 0% translated and 0% approved.