かつては女だったって英語でなんて言うの？

Talking about trans people can be a very difficult topic, especially across language and culture barriers. The best way to state that the person is trans is to say that they were born in a woman's body to note that previously they were a female. Then, you state that they identify as male so that your listener understands that they are trans.

I hope that this helps :)

Some people feel as though they do not belong in the body they where born in. They many feel as though they emotionally and psychologically belong as members the opposite sex. Such people then undergo medical treatment to acquire the physical characteristics of the opposite sex, being known as a transsexual.

In this example, it is referring to a woman who has undergone treatment to become a man.

Having a sex change is a huge moment in someones life and can make that person into the person they have always felt that they were. This is usually done with a mix of medical procedures and hormone therapy to become the opposite sex.

He had a sex change, he used to be female.
He had an operation to become a man.

All of these can be used to explain that someone is a transsexual. A transsexual is someone that permanently changes from one sex to another usually with operations and some people undergo hormone replacement therapy.

Well, in these days of hyper-sensitivity and political correctness, one can hardly say anything without offending someone somewhere. It seems we should ignore all differences even if they are blatantly obvious and a natural cause for either comment, concern or comedy. Until 2010, the word 'tranny' was a common informal term for transsexual in the UK, however several rights groups have outlawed the word and Facebook now categorises the word as a 'hate word'. Of course you could refer to their previous and present name as in the first example sentence - although probably, Dylan would be offended if people were referring to him in any way which might detract from his masculinity!

Some people feel as if they belong in the other sexes body so might want to transgender or change into the opposite sex - man becomes a woman or woman to become a man
He once was a woman or he used to be a female/woman refers to a man that has transgendered from a woman

In the current world it is difficult to stereotype a specific gender, now there are many, so it is hard to say 'He' or 'She'.
"They" Can be used to describe One or many people and a 'He' or 'She'.