hello: command not found

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

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

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/hello - from package: hello-traditional
/usr/bin/hello - from package: hello

You may install the required package with command apt-get install {package}

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