Don’t know if anyone ever resolved this issue but I just had the same problem after an upgrade. the file “mo.php” could not be found. It was supposed to be in the subdirectory “pomo” inside wp-includes.
I could log in using ftp and so I searched and found the “mo.php” file. Instead of wp-include/pomo it was in wp-include/wp-include. I had 2 wp-includes, one nested within the other.
Copied the nested wp-include that contained mo.php and 4 other files to my desktop and then created a directory called “pomo” in the original, outer wp-include. Did not delete the other sub “wp-include”. Everything appears to be working.
I had used an automatic install. I can’t tell if this is a bug or not, but it’s mighty strange.
