I'm generally an en_*.UTF-8 user (even tried en_DK.UTF-8 for a bit for a reason we'll come back to), so I don't have a complete picture of it and would have to go look at the documentation or source for that, but I'd expect

• documentation
• date formats: en_DK.UTF-8 should give you ISO8601-formatted dates, if I can't have that I at least want DD/MM/YYYY; the US-american nonsense is just plain unacceptable
• sorting: e.g. Norwegian will have …zæøå and expect aa to be sorted as å, the Swedes have …zåöä, the Germans …zäöü, the Turks will want ı and İ sorted and upper/lowercased correctly, and there are some options around how you deal with "foreign" letters and diacritics.
• Probably more stuff relating to LC_* that I can't think of off the top of my head

but in any case, an ls -l output should be different depending on your locale, and in ways you likely don't even think about as long as it looks normal.