\documentclass{standalone}

\documentclass{standalone}
\usepackage{luamplib}
\begin{document}
\begin{mplibcode}
def U primary s = if string s: decode(s) fi enddef;
vardef decode(expr given) =
save a,b,i,s,out; string s, out; numeric a, b, i;
out = ""; i=0;
forever:
i := i+1; s := substring(i-1,i) of given; a := ASCII s;
if a < 128:
elseif a = 194:
i := i+1; s := substring (i-1,i) of given;
elseif a = 195:
i := i+1; s := char (64 + ASCII substring (i-1,i) of given);
else:
s := "?";
fi
out := out & s;
exitif i >= length given;
endfor
out
enddef;
beginfig(2);
draw "café noir £2.50" infont "ptmr8r";
draw decode("café noir £2.50") infont "ptmr8r" shifted 12 down;
defaultfont := "pncr8r";
label.rt("café noir £2.50", 24 down);
label.rt(decode("café noir £2.50"), 36 down);
endfig;
\end{mplibcode}
\end{document}