Bind "/path/to/apertium-xsel" and "/path/to/apertium-xsel prev" to some global shortcut key, and get apertium translation on whatever you've selected with your mouse (if it's an URL, then it'll download the page, translate it and open that). The one with the "prev" argument will use your most recently used language pair, the other one pops up a …

apertium-xsel

#!/bin/bash

# This file gives you apertium translation on whatever you've selected
# with your mouse. When called with "prev" as an argument, it will use
# your most recently used language pair, without it will pop up a
# picker. It only uses installed pairs, ie. the ones listed by
# "apertium -l".

# Install dependencies on Debian/Ubuntu:
# $ sudo apt-get install xsel zenity
# Install dependencies on Arch Linux:
# $ sudo pacman -S xsel zenity

# And of course you need some apertium language pairs.

# chmod +x this file, then bind "/path/to/apertium-xsel" and
# "/path/to/apertium-xsel prev" to some global shortcut keys of your
# choice. In xfce, you can do this by opening the Settings Manager and
# selecting Keyboard and the Application Shortcuts tab and Add.


set -e -u

if command -V xdpyinfo &>/dev/null; then
    # Half of screen in either direction:
    width=$(xdpyinfo | awk -F' +|x' '/dimensions:/{print $3/2}')
    height=$(xdpyinfo | awk -F' +|x' '/dimensions:/{print $4/2}')
else
    width=600
    height=400
fi

recentfile=$HOME/.config/apertium-xsel.recent

mkdir -p $(dirname "${recentfile}")
touch "${recentfile}"
recent=$(tail -20 "${recentfile}")

if [[ $# -ge 1 && $1 = prev ]]; then
    pair=$(echo "${recent}"|tail -1)
else
    pair=$(
	cat <(echo "${recent}") <(apertium -l) |
	sort | uniq -c | sort -nr |
	sed 's/^ *[0-9][0-9]*  *//' |
	zenity --list --title "Apertium" --text "Select language pair" --column "pair" --hide-header --height "${height}" |
	sed 's/|.*//'		# work around zenity 3.8 bug #711251
    )
fi

echo ">>${pair}<<"
if [[ -n ${pair} ]]; then
    echo "${recent}" > "${recentfile}"
    echo "${pair}" >> "${recentfile}"
    if [[ $(xsel|head -1) =~ ^(file|https?):// && $(xsel|wc -l) -le 1 ]]; then
	tmpfile=$(mktemp -t apertium-xsel.XXXXXXXXXXX)
	curl "$(xsel)" >"${tmpfile}" &&	exo-open "${tmpfile}" || zenity --error --text "Couldn't download $(xsel)"
    else
	cat <(xsel | apertium "${pair}" 2>&1) <(printf "\n\n%30sOriginal:%30s\n") <(xsel) |
	fold -s |
	zenity --list --column "${pair}" --text "" --title "Apertium ${pair}" --hide-header  --width "${width}" --height "${height}" --cancel-label OK
    fi
fi