How to know if an keyboard/input related app is safe to use I am Vietnamese and I cannot type my language on my Android phone i.e. Samsung Galaxy S4

So I look for apps that help me to input Vietnamese. My issue is that I'm affraid the apps will bug me and know everything sensitive that I type.

How can I know if one such app is safe to use?

p.s. I add a sample of such apps as below - Laban Key; with the required permission(s)
You can't. A keyboard, by nature, is required to know everything you type, including your account credentials. Your best bet is to select keyboard coming from top developers.

My favorite? Multiling Keyboard, supports multiple languages (including Vietnamese), highly customizable, the only keyboard I know of that doesn't require Internet permission (so the keyboard can't send what you are typing on the fly).

Some other popular keyboard that support Vietnamese includes Swiftkey (free), Swype (paid).

As for Vietnamese products, Laban is fine (comes from Vinagame I think), Gotiengviet also was used by hundreds of thousands users without any incident.