Set As Default on any enabled language starts a confirmed change: Tringify first shows you exactly what the switch will do, and only applies it once you confirm.
What changes
The promoted language becomes your store's new source language — content is treated as authored in it from then on. Any existing translations for that language are deleted, because it's now the origin rather than a translation. Your primary storefront also switches to it. This cannot be undone.
Requirements
The language must already be in your enabled set. If you're at your plan's language limit, remove a language first, then change the default.