At first thought maybe Pebble uses Google voice API but after a quick check, I got this news about PT voice engine and voila here is their demo: it supports many more languages than the Pebble App allow us to choose. Now the tweak begins.
Having used adb shell on my Android phone, I figured out that the Pebble Time app has saved the configuration in this SQLite file /data/data/com.getpebble.android.basalt/databases/pebble, in table "preferences" and the key is "voice_language". Now we can change the language to the one which matches with this document ( for example "eng-USA", "vie-VNM" ) and reboot.
Ehhmm, too much technical things? To make it easier for most users I created a small Android app which you can download here. (Remember, you need rooted Android phone to make this work, I will public source code to Github later).
When you use custom voice language and open Pebble Time app --> Settings, It will crash because it doesn't understand that language. All you need to do is use my app and change back to English, everything will be normal again. After that you can select back to your language with my app. New Pebble app version now worked ok with this tweak.
2016/05/29: I updated my tweak app to support MarshMallow and environment doesn't have busybox.