\documentclass[spanish,ngerman,frenchb,english]{article}
%\documentclass[spanish,ngerman,english,frenchb]{article}
%\documentclass[spanish,frenchb,english,ngerman]{article}
%\documentclass[frenchb,english,ngerman,spanish]{article}
\usepackage[latin1]{inputenc}
\usepackage{url}
\usepackage{babel}

%% Load package with fixed language or flexible language:
%\usepackage[languagenames]{babelbib}[2003/04/21]
\usepackage[languagenames,fixlanguage]{babelbib}[2003/04/21]

%% force a fall-back language other than the document's language
\setbtxfallbacklanguage{ngerman}

%% Switch off to print ISBNs and ISSNs:
\btxprintISBN{false}
\btxprintISSN{false}

%% Auto format date strings in 'urldate'
\IfFileExists{isodate.sty}{%
  \usepackage[short]{isodate}
  \setbibliographyfont{urldate}{\printdate}
}{\typeout{Package isodate not available. Skipping that functionality}}

%% Change font for some parts of the bibliography:
%\setbibliographyfont{name}{\textsc}
%\setbibliographyfont{lastname}{\textsc}
%\setbibliographyfont{title}{\textbf}
%\setbibliographyfont{etal}{}
%\setbibliographyfont{volume}{\textsc}
%\setbibliographyfont{journal}{\textsc}

%% Change language for bibliography when option `fixlanguage' is chosen:
%\selectbiblanguage{ngerman}

%% Change or add keywords to specific languages:
%\declarebtxcommands{english}{%
%  \def\btxphdthesis#1{\foreignlanguage{english}{Ph.D.\ thesis}}%
%}
%\declarebtxcommands{ngerman}{%
%  \def\btxphdthesis#1{\foreignlanguage{ngerman}{Doktorarbeit}}%
%}

%% Use the next line to avoid the babamspl style to print out the
%% language names:
% \newcommand\btxprintamslanguage[1]{}

\begin{document}
If you change \verb|\usepackage[languagenames,fixlanguage]{babelbib}| to
\verb|\usepackage[languagenames]{babelbib}|, the
bibliography will change the language citation-dependent.
Then, an error will appear at citation in a language that is not yet
available. 
This is normal and can be avoided by helping me to add that language.

You may change the \verb|\bibliographystyle| line to test all styles.

There are most of the available commands to change anything in the
source code, but commented out. 
Remove the \%\ in order to see the effects.

\nocite*

%% Choose one BibTeX style:
%\bibliographystyle{plain}
%\bibliographystyle{bababbrv}
%\bibliographystyle{babplain}
%\bibliographystyle{babplain-lf}
%\bibliographystyle{babplain-fl}
%\bibliographystyle{babalpha}
%\bibliographystyle{babunsrt}
\bibliographystyle{babamspl}

\bibliography{babelbibtest}

\end{document}
