This is a follow-up to T320523#8504444

Currently, if an unsupported language is passed in lang="" then an error popup is shown that says something like "Language cmn is not supported by Phonos. The following possibly-related languages are supported: cmn-CN and cmn-TW."

The list of suggested language codes is created by looking for substrings that match the requested on. This process is not great, because it results in incorrect suggestions such as af-ZA (Afrikaans) being suggested for za (Zhuang).

Rather than provide a list like this, the error message should link to a new special page (e.g. Special:Phonos) where all available languages are listed, perhaps also along with example code and samples of the audio. This could also later be used to provide samples of different voices (if T324738 is done).