Hi Ana,
The functions don't use text to speech, they play the appropriate system prompts for the currently executing language of the flow. The note in the the expression help was explaining that the example audio is what is heard when the current language of the flow is en-US.
So when the flow is using en-US, the caller would hear the following system prompts:
daysofweek_Monday ("Monday")
month_november ("November")
num_13_ordinal ("thirteenth")
num_2 ("two")
num_thousand ("thousand")
num_17 ("seventeen")
However for Portuguese the caller would hear these system prompts:
daysofweek_Monday ("segunda-feira")
num_13 ("treze")
month_of_november ("de Novembro")
preposition_de ("de")
num_2 ("dois")
num_thousand ("mil")
conjunctive_and ("e")
num_17 ("dezessete")
Notice that audio for each language has the appropriate grammar, i.e. in en-US it uses "November" but in pt-BR it uses "de Novembro".
This feature is called runtime data playback. It's not supported for every language; it works in Portuguese (pt-BR) but not Belgium. When you add a supported language to a flow, Architect will put a * next to it if runtime data playback is not supported.
https://help.mypurecloud.com/articles/supported-languages-architect/
https://help.mypurecloud.com/articles/purecloud-supported-languages/
https://help.mypurecloud.com/articles/choose-supported-languages/