NAKL is an open source Vietnamese input keyboard for Mac OS. Its engine is based on XVNKB, a well known input keyboard on Linux. NAKL is fully compatible with all the Mac OS versions from 10.6 (Snow Leopard) to 10.12 (Sierra).
Please report issues on the GitHub issue tracker.
This problem occurs from version 10.8 (Mountain Lion) onward. To fix this, open System Preferences -> Security & Privacy -> in General tab, tick on option Anywhere for Allow applications downloaded from.
In order to use this application, the option "Enable access for assistive devices" must be enabled. Open System Preferences -> Universal Access (for Mac OS 10.8 Moutain Lion, open System Preferences -> Accessibilty -> Keyboard), tick on "Enable access for assistive devices".
Open System Preferences > Security & Privacy > Privacy > Accessibility. Click the padlock icon in the lower left corner and enter your admin password. Once it's unlocked, drag NAKL app from Finder or from Applications panel into the apps list in the System Preferences window then tick the checkbox.
If NALK is running, please quit and open it again.