Selamat Sejahtera

Selamat malam kawan-kawan. Nama saya Dana. Saya suka bercakap dalam bahasa Melayu. Bahasa Melayu cantik.

Selamat Sejahatra-selamat sejahtera

Selamat malam kawan kawan. Nama saya Dana. Saya suka cakap bahasa Melayu. Bahasa Melayu cantik.

Selamat Sejahatra

Selamat malam kawan kawan. Nama saya Dana. Saya suka cakap bahasa Melayu. Bahasa Melayu cantik.

1) Selamat Sejahtera.
Nothing to be explained. You just have to remember the spelling. 

2) Kawan-kawan.

Don't forget to add " - " there because you are doubling a word that you want to change it into plural.

3) Bercakap
Don't forget to put the affix " ber "
It's important.
If you want me to list all the Malay affixes, message me.

4) Cantik
Yes, I agree with you, Bahasa Melayu is beautiful. 
But, in this case, you should have said, " Bahasa Melayu sangat ( very ) indah ".
Indah and Cantik mean " beautiful "
Cantik is more on person appearance, while Indah is more to Nature and Flora or easy to say, it's for God creature.
But, you may also say " indah " instead of " cantik " when you are telling about languages.
I am not really sure if you can use " cantik " or not, but If me, I prefer to say " indah "