changing u. a. of German to et. al

The astron style, as explained in this answer, worked for me.

Following worked for me with APA style:

documentclass{article}
usepackage{csquotes}
usepackage[ngerman]{babel}
usepackage[
    backend=biber,
    bibwarn=true,
    bibencoding=utf8,
    sortlocale=de_DE,
    style=apa,
]{biblatex}
DeclareLanguageMapping{ngerman}{ngerman-apa}
DefineBibliographyStrings{ngerman}{
   andothers = {{et,aladddot}},
}
addbibresource{Bibliography.bib}

begin{document}
  Sample: textcite{beck:2001}
end{document}

Bibliography.bib:

@misc{beck:2001,
  Author    = {Beck, K. and Beedle, M. and van Bennekum, A. and Cockburn, A. and Cunningham, W. and Fowler, M. and Grenning, J. and Highsmith, J. and Hunt, A. and Jeffries, R. and Kern, J. and Marick, B. and Martin, R.C. and Mellor, S. and Schwaber, K. and Sutherland, J. and Thomas, D.},
  Title     = {Manifesto for Agile Software Development},
  Year      = {2001},
  URL       = {http://www.agilemanifesto.org/},
}

Credits go to this guy

Simply changing the pushed standard texts defined in dinat.bst will work.

I downloaded the dinat.bst file: https://ctan.org/pkg/dinat

and modified

FUNCTION {push.ua} { "u.,a." }

FUNCTION {push.und} { "und" }

into

FUNCTION {push.ua} { "et ,al." }

FUNCTION {push.und} { "and" }

Then, put this file in the folder of the .tex file.

I had the same problem with similar style, as you are having. The following code worked for me to get 'u.a.' to 'et al.':

usepackage[sort&compress, square]{natbib}
usepackage{har2nat}
bibliographystyle{agsm}

begin{document}
   citep{Author.2020}
   bibliography{bib_file}
end{document}

Or you can change the entry in your .bib file, i. e. put curly brackets:

AUTHOR       = "{Beck, K. et al.}"