apertium-validate-modes: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/apertium-validate-modes - from package: apertium

You may install the required package with command apt-get install {package}
Error in other languages:
apertium-validate-modes: bevel nie gevind nie
apertium-validate-modes: comandă negăsită
apertium-validate-modes: comando não encontrado
apertium-validate-modes: commande introuvable
apertium-validate-modes: command not found
apertium-validate-modes: käsku ei ole
apertium-validate-modes: không tìm thấy lệnh
apertium-validate-modes: komanda nerasta
apertium-validate-modes: Kommando nicht gefunden.
apertium-validate-modes: kommandot finns inte
apertium-validate-modes: komut yok
apertium-validate-modes: nie znaleziono polecenia
apertium-validate-modes: níor aimsíodh an t-ordú
apertium-validate-modes: no se encontró la orden
apertium-validate-modes: no s'ha trobat l'ordre
apertium-validate-modes: opdracht niet gevonden
apertium-validate-modes: parancs nem található
apertium-validate-modes: perintah tidak ditemukan
apertium-validate-modes: príkaz nenájdený
apertium-validate-modes: příkaz nenalezen
apertium-validate-modes: команда не найдена
apertium-validate-modes: командата не е открита
apertium-validate-modes: 命令找不到
apertium-validate-modes: コマンドが見つかりません