"His daughter speaks seven languages."
Translation:Seine Tochter spricht sieben Sprachen.
Why is it Sprachen and not Sprächen?
Plurals cannot generally be predicated; you generally have to learn them along with the singular form and the gender.
Some plurals involve umlaut, others do not. Sometimes, even both possibilities are possible... (e.g. der Wagen has the plural die Wagen in Germany but die Wägen in Austria).
why is 'redet' not acceptable ?
For the same reason that we wouldn't say "he talks three languages".
"talk" and "speak" are often interchangeable when you're just talking about conversation, as are reden and sprechen.
But in the context of "speaking a language", we only use "speak" / sprechen.
(At least in the standard language. Dialects or other varieties of High German might be different - for example, I think redst du yiddish would be fine in Yiddish and I wouldn't be surprised if, say, Pennsylvania Dutch people use a variant of reden.)