Gedit is a simple GUI code / plain text editor (written in C/Python)
There's an apertium translate plugin part of the official gedit-plugins package, should be possible to install with
sudo apt-get install gedit gedit-plugins
if your Debian/Ubuntu is new enough.
To use gedit for editing Apertium XML:
sudo apt-get install gedit gedit-developer-plugins
There's an XML validation plugin at https://launchpad.net/gedit-xmltools but it only works with gedit2