I was going to suggest connecting the plugin you have to OnFileManagerDirCreate too and borrow code from the upload section, but OnFileManagerDirCreate makes no use of it's return value.
You could put in a feature request for adding an OnBeforeFileManagerDirCreate event. It should be a pretty simple addition to the file mentioned above.
Another possibility would be to attach your existing plugin to the OnFileManagerDirCreate event too and add code to rename the directory using $source->renameContainer().
It would look something like this (untested):
/* code to change the value of the $name variable here, based on the code in the current plugin */
$source->renameContainer($directory, $name);
If you want to paste the code of the filetranslit plugin here, we might be able to suggest the proper rewrite.
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