documentclass[a4paper,11pt]{article}

usepackage[english,greek]{babel}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{graphicx}
usepackage{xcolor}
usepackage{tgtermes}
usepackage{amsfonts}

newcommand{lat}{selectlanguage{english}}
newcommand{gre}{selectlanguage{greek}}