The example was installing babel-spanish, if you replace that with hyphen-icelandic what happens?
rstudio-user@application-1753700-deployment-4935334-b4b9t:/cloud/project$ tlmgr init-usertree
(running on Debian, switching to user mode!)
tlmgr: action init-usertree returned a warning.
rstudio-user@application-1753700-deployment-4935334-b4b9t:/cloud/project$ tlmgr option repository ftp://tug.org/historic/systems/texlive/2015/tlnet-final
(running on Debian, switching to user mode!)
tlmgr: setting default package repository to ftp://tug.org/historic/systems/texlive/2015/tlnet-final
rstudio-user@application-1753700-deployment-4935334-b4b9t:/cloud/project$ tlmgr --usermode --no-persistent-downloads install hyphen-icelandic
(running on Debian, switching to user mode!)
TeX Live 2015 is frozen forever and will no
longer be updated. This happens in preparation for a new release.
If you're interested in helping to pretest the new release (when
pretests are available), please read http://tug.org/texlive/pretest.html.
Otherwise, just wait, and the new release will be ready in due time.
tlmgr: package repository ftp://tug.org/historic/systems/texlive/2015/tlnet-final
[1/2, ??:??/??:??] install: hyph-utf8 [1002k]
[2/2, 00:02/00:02] install: hyphen-icelandic [1k]
tlmgr: package log updated: /home/rstudio-user/texmf/web2c/tlmgr.log
running mktexlsr ...
done running mktexlsr.
regenerating language.dat
TLUtils::_create_config_files: /home/rstudio-user/texmf/texmf-dist/tex/generic/config/language.us: head file not found,ok in user modeCannot open /var/lib/texmf/tex/generic/config/language.dat for writing: Permission denied at /usr/share/texlive/tlpkg/TeXLive/TLUtils.pm line 2748.
I have no means of testing the success or not, though I am happy to play around if you share a reprex.