\documentclass{article}
\usepackage{xepersian}
\makeatletter
\AtBeginDocument{%
\bidi@patchcmd{\@LTRfootnotetext}{\latinfont}{\sffamily\bfseries}{}{}
\bidi@patchcmd{\@RTLfootnotetext}{\persianfont}{\bfseries\iranicfamily}{}{}
}
\bidi@patchcmd{\lr}{\latinfont}{\bfseries\sffamily}{}{}
\makeatother
\begin{document}
\lr{This is a test.}
\LTRfootnote{This is a test.}\RTLfootnote{این یک زیرنویس است.}
\end{document}