OK, I started (committed) some initial AI use of translatable UserStrings. I believe the manner of use should be detectable by the scanning for 'UserString' intended by adrian broher. This initial use is limited to an expanded roster of Empire Capitol planet names. You may discover the new names via observation or neatly aggregated in the Pedia under Game Concepts->AI Aggression Levels.
As mentioned in a Translators Note in the EN stringtable, translators are free to specify more or fewer Capitol Planet Names than are currently listed in the EN stringtable, subject to the list formatting requirement identified there.
While writing this post I realized that my internal English->English translator seemed to have gone on the fritz, and I used "CITY" in the stringtable labels rather than "CAPITOL" That is now corrected.
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I like the list-of-strings in a stringtable entry mechanism used... For a while I've wanted to replace starnames.txt with entries in the stringtable so they can be localized when appropriate. But I never did so because it seemed like it would require 500+ new stringtable entries for all the system names. This would seem like a good way to avoid that. It would just require a bit of extra code to break the from-stringtable list of strings into the individual system names. Possibly the existing code that does this for the current starnames list would work; I haven't looked.
That is now corrected.