You need to redefine the list format. To insert the "in" in the appropriate place we can slightly modify the macro responsible for the list delimiters.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear, backend=biber]{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{appleby,
author = {Humphrey Appleby},
title = {Über die Bedeutung des Berufsbeamtentums},
language = {german},
date = {1980},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\newbibmacro*{list:in:delim}[1]{%
\ifnumgreater{\value{listcount}}{\value{liststart}}
{\ifboolexpr{
test {\ifnumless{\value{listcount}}{\value{liststop}}}
or
test \ifmoreitems
}
{\printdelim{multilistdelim}}
{\lbx@finallistdelim{#1}}}
{\bibstring{in}\addspace}}
\DeclareListFormat{language}{%
\usebibmacro{list:in:delim}{%
\ifbibstring{#1}
{\bibxstring{#1}}
{\ifbibstring{lang#1}
{\bibxstring{lang#1}}
{#1}}}%
\ifbibstring{#1}
{\bibstring{#1}}
{\ifbibstring{lang#1}
{\bibstring{lang#1}}
{#1}}%
\usebibmacro{list:andothers}}
\begin{document}
\cite{appleby}
\printbibliography
\end{document}

In
before thelanguage
, I'll assume you need to change what you have so far only in this regard. Then use\DeclareListFormat{language}{In #1}
. Otherwise, as moewe said, please provide a MWE.\DeclareListFormat{language}{In #1}
is not a good solution. It does not localise the language names.