करोड - karor is equal to 10 million. you are right, in sanskrit/ hindi there is no single word for a million. a thousand thousand translates to सहस्त्र सहस्त्र. करोड़पति karorpati does not literally translate to "millionaire". However, when you say millionaire in english I am sure you count everybody who has from 1 million to 999 million as a millionaire. if you translate that sense to hindi, karorpati can be understood as millionaire, or it can just be taken to mean "very rich"
इकत्तीस ikattīs / इकतीस iktīs - 'thirty-one' - these are just regional variations of pronunciation. i am from the eastern part of India and I pronounce it as इकतीस iktis, whereas people from the northern parts of the country pronounce it as इकत्तीस ikattis. the answer to your question - usage varies by region.
the same answer applies to the other numbers
चौवालिस
cauvālīs - eastern region
चवालीस
cavālīs - northern/ western region
तिरपन
tirpan - 'fifty-three' - eastern region
'trepan'. or 'tirepan' is used more in the northern/ western regions. to confuse you a little bit, while spoken variations exist, most written/ published books go with the northern variant and therefore the people from other parts of the country are more familiar with the northern variant. to give you an example from south-eastern usa, people say "y'all " but when written in a book it is written as "you all". this doesn't mean that the spoken colloquial version is wrong.
alok
http://kumar-alok.blogspot.com