> Should I change lang="en" into lang="th" or should I leave it like it is
The `lang` attribute describes the language the document is written in. If you are writing in Thai, you should not claim you are writing in English.
> How do I add Thai langauge to a website?
Just type it.
> And what about charset? Is there another charset for Thai language?
There are plenty of character encodings out there. Different ones support different sets of characters. UTF-8 supports all the characters you need for just about every language out there and is a good default choice.
You might get [better performance out of TIS-620][1] but note that you'll need to actually save the file as TIS-620. Just telling the browser you are using it will not work.
> How to use Thai web font on a website?
The same way you use any other web font.
[1]: http://www.micro-isv.asia/2009/03/why-not-use-utf-8-for-everything/

> Should I change lang="en" into lang="th" or should I leave it like it is
The `lang` attribute describes the language the document is written in. If you are writing in Thai, you should not claim you are writing in English.
> How do I add Thai langauge to a website?
Just type it.
> And what about charset? Is there another charset for Thai language?
There are plenty of character encodings out there. Different ones support different sets of characters. UTF-8 supports all the characters you need for just about every language out there and is a good default choice.
You might get [better performance out of TIS-620][1] but note that you'll need to actually save the file as TIS-620. Just telling the browser you are using it will not work.
> How to use Thai web font on a website?
The same way you use any other web font.

[1]: http://www.micro-isv.asia/2009/03/why-not-use-utf-8-for-everything/