hugo-theme-bootstrap | A fast, responsive, multipurpose and feature-rich Hugo theme | Theme library

(TLDR - The site author set-up the author data as 1 config file - i.e. Theme is built for 1 author for the entire site).

Hello Pascal, so, really appreciated you took the time to answer me. To clarify:

1. This isn't a "hugo" thing, but the way this chap built this theme.

2. You are going to have to modify his theme (or reach out to him). The specific file in question, as you point out, I believe is profile.html in sidebar:

   {{- if .Site.Author -}}
   {{- $layout := default "" .Site.Author.params.layout -}}
   {{- if eq $layout "compact" -}}
   {{- partial "sidebar/profile/compact" . -}}
   {{- else -}}
   {{- partial "sidebar/profile/default" . -}}
   {{- end -}}
   {{- end -}}

If you follow the various partials this calls, i.e. down the rabbit whole you will find an example (this is one of many), like you describe and quote in your question, it has the specific comment {{ - with .Site.Author - }} meaning, it's looking for exactly 1 file, with a specific name.
[https://gohugo.io/content-management/multilingual/][1]
covers how to set-up a hugo multilingual - and there is SUPPOSED to be 1 Config file per site (which has it's own meaning in Hugo - see above page) Point being, this is a theme issue, and I would suggest asking the theme creator to adapt edit. My suggestion would be if the theme creator isn't helpful - would be to remove the dependency for the TOML file, and simply reference a headless bundle which has the author data, and as the theme has i18n, integrate the two (see above link) so that their is a headless bundle per language.