'Hello sexy' in Welsh, is actually pretty similar to how we say it in England. This is unusual as the Welsh language is quite different to English.
This is the Welsh translation: Helo sexy
The only difference to English here is the one 'l' in hello.
If you want a similar phrase, there are a couple of alternatives that you could use.
'I like you' - Cara 'ch
'I want you'- Fi angen 'ch
'Kiss me'- Chusana 'm
'You are beautiful'- Ach 'n arddun
'You are so sexy'- Ach fel sexy
There is actually a simple way that you can translate passages from one language to another yourself, however you need to be careful as it is not always 100% accurate.
There are websites that you can use such as www.online-translation.com, or most of the most common search engines like google and bing offer free translation services. All you you have to do is type out the passage you want translating into one of the boxes on the website, and your translation will appear in the second box.