Once you've replaced a Text property with an Internationalized String if you edit it inline in Hierarchy or Content and enter say "whatever" then you're back to a regular value, the information it was an I18N key is lost.

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Set text as Internationalized String with cog menu for text in Inspector.
In Hierarchy or Content set the value to "something" ==> what is saved is "something" as I was expecting "%something".
IMO the inline editing should preserve the fact what is edited is a key, not a regular plain string.