droplang: 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 CentOS possible default full paths are:

/usr/bin/droplang - from package: postgresql

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/droplang - from package: postgresql-client-common
/usr/lib/postgresql/9.4/bin/droplang - from package: postgresql-client-9.4

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

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