Okay, Cecil explains at the time of planning DC the two letters were interchangeable. That’s a good explanation for a city planned in 1791. But the question also noted that the same missing letter is present in Anchorage, a city planned in the 1920’s. So how late did the practice of skipping the letter J last until?
Perhaps Anchorage was deliberately copying the Washington street names?
As far as I remember my Latin lessions in school, there was no J in the Latin alphabet - hence Iulius Caesar. The classical Latin had only 21 letters; initially C was also standing in for G as in Caius (=Gaius) Iulius Caesar. The letters were A B C D E F (G) H I K L M N O P Q R S T V X.
Pit
Y and Z were added late for transliteration of Greek.
C was originally pronounced G (which is why it is the third letter, as in Greek and the West Semitic alphabets). Only a little over 2,000 years ago, it started being pronounced like “K”, and the new letter G was invented to cover the relatively small number of words that still had the old pronunciation. It was inserted in the place of Z, which Latin did not need, though, as remarked above, it was later put back, though at the end.
Well into the 20th century, J was used in Italian only for spelling “II” as “IJ”, which was regarded as more decorative. It’s not used in standard Italian at all, these days.