Hallo, I had the problem, that some of my environments appeared in the wrong language and I couldn't change the language all at ones.
Now I want to explane how to solve the problem: See how lyx names the environment in the source code. For example there's a code like the following generated by Lyx (be sure you can see the hole source code: see check mark on the right side): \newtheorem{rem}[thm]{Remark} If you want that remark is written in another language, for example in German add your own code (Document->Settings->preambel) as follows: \let\rem\undefined \let\endrem\undefined \theoremstyle{definition} \newtheorem{rem}[thm]{Bemerkung} \ifx\proof\undefined\ \newenvironment{proof}[1][\proofname]{\par \normalfont\topsep6\p@\@plus6\p@\relax \trivlist \itemindent\parindent \item[\hskip\labelsep \scshape #1]\ignorespaces }{% \endtrivlist\@endpefalse } \providecommand{\proofname}{Proof} \fi This works in most cases. It didn't work for the Lyx code \newtheorem{thm}[thm]{Theorem} and I think this is because thm has different meanings: At first it is an environment and second it has something to do with the package name (amsthm). When I made the same with thm my computers (tested at two different computers with 2 different OS) computed until I stopped them. When I took the thm in []-brackets out of my code \newtheorem{thm}{Satz} Lyx gave some “error message”. Thanks for help, Günter! Mike Bonhoff -- NEU: FreePhone - kostenlos mobil telefonieren! Jetzt informieren: http://www.gmx.net/de/go/freephone