automatically switch between voices depending on languages

Some users would often switch between a couple of languages, and more than often one cannot use the same voice for the different languages. We'd need to automatically switch between voices in such a case. There are several things:

- one may have to use a different module, in case a given module provides very good support for one language, and another module provides very good support for another language
- the client could just tell the language, and let speechd determine which module should be used
- speechd could be configured to automatically switch language depending on the character set being used, without the need for the screen reader to tell so.

No file or test is identified; begin by examining the client language signal, speechd module selection, and configuration options described in the issue. Define done as automatically selecting appropriate voices or modules when languages change, including the proposed client-provided language and character-set cases.

Written by the indexing model from the issue text.