Yes, speach-synth is a powerful feature, regardless the language. Everybody should be able to learn some basic words as: Volt / Ampere / RSSI and so on.
As the announcement will only be needed for some serious events, like battery exhaustet or out of range.
Otherwise a couple of different beeps, or even better the vibration alert, in different intensity or rythm can be used to indicate the wanted alerts.
I intend to use the system not in an airplane, but in an fast electric boat (100+ mph), which can "stand" just on the water if not driving - a plane will surely not
So i can inform myself how much mAh´s allready used, or some other information i want to have. When driving/flying there is no possibility to look for the display i agree, so the acoustic or vibration alert is much more suitable to inform the pilot.
To have it all together in one "toy" looks to be more attractive, as you can´t forget to take all boxes and cables around with you when going to have a flight, just take the radio, the model, some (hopefully charged) lipo´s and off you go.