Defining \captionmainlanguage after loading the bicaption package does not work
Yes, it is documented that it should be defined before loading the package, but doing it afterwards should work, too, or at least issue a warning or error message.
Example document:
\documentclass[a4paper]{article}
% Load the bicaption package with 2nd language set to
% "english"
\usepackage[lang=english]{bicaption}
% Set "german" as main bi-caption language
\newcommand\captionmainlanguage{german}
% Declare an own language switching mechanism
% for bi-captions (instead of using babel)
\renewcommand\selectcaptionlanguage[2]{%
\csname captions#2\endcsname}
\newcommand\captionsgerman{%
\renewcommand\figurename{Abbildung}%
% ...
}
\newcommand\captionsenglish{%
\renewcommand\figurename{Figure}%
% ...
}
\begin{document}
\begin{figure}
\centering
A placeholder for an image or whatever
\bicaption{Deutscher Text}{English text}
\end{figure}
\end{document}