documentclass{article}

usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}

makeatletter

begin{document}

expandafterdefcsname c cv c'eendcsname{I am weird}
csname c cv c'eendcsname:
expandafterstringcsname c cv c'eendcsname

% with inputenc:
defäöü{Me too}
äöü:
% stringäöü does not work
expandafterstringcsname äöüendcsname

end{document}