How can I change the default language so that instead of my signatures I get something else instead of "Table" or "Figure"?
You can change the language of the document using the package babel. Headings will be updated automatically.
babel
As a bonus, you will receive the correct placement for the selected language.
You can also override \tablenameand \figurename.
\tablename
\figurename
\usepackage[francais]{babel}
You can override \figurenameand \tablenameusing \renewcommand. For example, you can do this for the Swedish language:
\renewcommand
\renewcommand{\figurename}{Figur} \renewcommand{\tablename}{Tabell}