No it is not a process. It is just a notation.
RUP(rational unified process) is process that takes UML as
an input.

It is not a process, its like Modeling i.e before starting 
the code if you want to know the high level design then we 
can use UML. 

After seeing the high level if there any modifications are 
required then we can make. 
Using this we can design the class 
level,usecase,Sequencial,Collabration etc.,

No, UML is just a notation Lnguage. NOt a programming 
language.

UML cannot be used for interface design, instead a good UI 
design is based on Usability practice or by following 
principles of user interface design.

You can design _model_ of pretty much anything you want 
with UML. You cannot design anything with UML.
User interface is just an interface of the system. You can 
use UML to reveal functional, static and dynamic aspects of 
user interface from model perspective. You can specify 
workflow, form contents, sequence of user actions, 
deployment aspects, etc. using UML.
Actual _implementation_ of user interface, including forms 
layout, design, internationalization, etc are out of UML 
scope.
The answer is: we can only model the interface, not design 
it.

No, Ther's lot of activities involved in Project SUccess. 
UML actually give the better way of modelling your design. 
Once your design is in healthy condiction, then obvisousily 
your project will get succeed.

Definetly not!

There is no any thing alone that can gaurantee project 
success, a successful project is attributed to many factors
such as requirement understaning/communication, 
understanding users, time & budget constraint, defined 
process etc.