Adobe Reader MUI is Adobe's Multilingual User Interface version. This product is capable of installing any combination of supported languages. So, for example, you can install French, German, and Spanish simultaneously and then have the capability toswitch among those languages within Adobe Reader. The user interface changes to the appropriate language on the fly.
So if you have MUI Reader 9.3.3(and NOT normal Reader) in your machine and want to update it to 9.3.4 choose Adobe Reader MUI 9.3.4 update - multiple languages otherwise choose Adobe Reader 9.3.4 update - multiple languages (for normal Reader).
To check what you have in your machine, Goto Control Panel>Add or Remove Programs and check the entry of Adobe Reader.
For MUI reader it will be
"Adobe Reader 9.3.3 MUI"
and for normal Reader it will be
"Adobe Reader 9.3.3"