This is a quick reference to documents discussing specific problems of HTML authoring in some languages. It is intended to suggest some starting points, with no attempt to comprehensiveness, in language coverage or in the resources for any particular language. For generalities, see Using national and special characters in HTML. The references below discuss e.g. specific 8-bit codes suitable for the individual languages, needed when using the "simple method" described in that document.
For European languages written using a Latin script, see Coverage of European languages by ISO Latin alphabets.
Hint: To see how others have authored Web pages in
some particular language, you could take a look at
which is a directory of pages by language.
You probably need to look at the HTTP headers and peek at the source code
to learn how the pages have been written.
The methods used might not
be optimal, and they might even be incorrect in different ways, but
at least you could get an idea of the situation.