% set default size and document class
\documentclass[12pt]{extarticle}

% handle multiple languages
\usepackage{polyglossia}

% set main language
\setmainlanguage{\mainlanguage}

% language commands

\newcommand{\bi}[2]{%
\expandafter\ifstrequal\expandafter{\mainlanguage}{en}%
{\ifstrempty{#1}{\ifstrempty{#2}{}{TRANSLATE\_ME!}}{#1}}%
{\ifstrempty{#2}{\ifstrempty{#1}{}{TRADUISEZ\_MOI\_!}}{#2}}%
}

\newcommand{\en}[1]{\textenglish{#1}}

\newcommand{\fr}[1]{\textfrench{#1}}