A Unicode encoding can support many languages and can accommodate pages and forms in any mixture of those languages. Its use also eliminates the need for server-side logic to individually determine the character encoding for each page served or each incoming form submission. This significantly reduces the complexity of dealing with a multilingual site or application.

A Unicode encoding also allows many more languages to be mixed on a single page than almost any other choice.

It is not much of an issue to move to Unicode these days.

Note that although there are other multi-script approaches (such as ISO-2022), Unicode generally provides the best combination of extensibility and script support.