csquotes and foreignlanguages: setting the outer language for quotation marks

If I understand correctly this should be enough:

DeclareQuoteStyle{french}
  {textquotedblleft}
  {textquotedblright}
  {textquoteleft}
  {textquoteright}

You can change the inner quotation marks to «...» with this:

DeclareQuoteStyle{french}
  {textquotedblleft}
  {textquotedblright}
  {guillemotleft}
  {guillemotright}

So that this code

 foreignquote{french}{Ceci est en français: enquote{Il l'a dit}}

gives:

I could not find an official interface on the csquotes side that lets you do this, but there are some internal macros that you can redefine to let csquotes first produce the quotation marks and only then switch the language.

documentclass[11pt,a4paper,french,english]{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage[autostyle=true]{csquotes}

makeatletter
%  {<init>}{<endinit>}{<text>}
longdefcsq@quote@i#1#2#3{%
  csq@bqgroupcsq@oqopen@i#1#3#2csq@qclose@i{}}
longdefcsq@quote@ii#1#2#3{%
  csq@bqgroupcsq@iqopen@i#1#3#2csq@qclose@i{}}
makeatother

begin{document}
  In an English paper, if I quote something that is in French,
  I have guillemets: foreignquote{french}{Ceci est en français.}
  
  I am looking for enquote{selectlanguage{french}Ceci est en français.}
end{document}

If you want to reset the quotation mark level so that the foreign-language quotes start at the outer level, you need to dig a bit deeper.

The following solution should also address most block language switching commands.

documentclass[11pt,a4paper,french,english]{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage[autostyle=true]{csquotes}
usepackage[backend=biber, style=authoryear]{biblatex}

addbibresource{biblatex-examples.bib}

makeatletter
newcountsaved@csq@qlevel

newcommand*{tcsq@saveandreset@qlevel}{%
  saved@csq@qlevelcsq@qlevel
  csq@qlevelz@}
  
newcommand*{tcsq@restore@qlevel}{%
  csq@qlevelsaved@csq@qlevel}

%  {<init>}{<endinit>}{<text>}
longdefcsq@quote@i#1#2#3{%
  csq@bqgroupcsq@oqopen@i#1#3#2csq@qclose@i{}}
longdefcsq@quote@ii#1#2#3{%
  csq@bqgroupcsq@iqopen@i#1#3#2csq@qclose@i{}}


defcsq@fquote@i#1{csq@quote@i{csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}}
defcsq@fquote@ii#1{csq@quote@ii{csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}}
defcsq@hquote@i#1{csq@quote@i{csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}}
defcsq@hquote@ii#1{csq@quote@ii{csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}}

% {<init>}{<endinit>}{<qopen>}{<citehook>}{<cite>}{<punct>}{<text>}{<apunct>}<tpunct>
longdefcsq@tquote@i#1#2#3#4#5#6#7#8#9{%
  begingroup
  csq@setsfcodes
  edefcsq@tempa{%
    unexpanded{%
      mktextquote
      {#3}%
      {#1#7}%
      {#2csq@qclose@i{}}%
      {#6}{#8}}%
    {ifblank{#5}
       {}
       {unexpanded{csq@switchlang{#4{#5}}}}}}%
  csq@bqgroupcsq@tempa#9%
  endgroup}

defcsq@ftquote@i#1{%
  csq@getcargs{csq@tquote
    {csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}
    {csq@oqopen@i}}}
defcsq@ftquote@ii#1{%
  csq@getcargs{csq@tquote
    {csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}
    {csq@iqopen@i}}}

defcsq@htquote@i#1{%
  csq@getcargs{csq@tquote
    {csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}
    {csq@oqopen@i}}}
defcsq@htquote@ii#1{%
  csq@getcargs{csq@tquote
    {csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}
    {csq@iqopen@i}}}

defcsq@ftcquote@i#1{%
  csq@getccargs{csq@tquote
    {csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}
    {csq@oqopen@i}}}
defcsq@ftcquote@ii#1{%
  csq@getccargs{csq@tquote
    {csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}
    {csq@iqopen@i}}}
  
defcsq@htcquote@i#1{%
  csq@getccargs{csq@tquote
    {csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}
    {csq@oqopen@i}}}
defcsq@htcquote@ii#1{%
  csq@getccargs{csq@tquote
    {csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}
    {csq@iqopen@i}}}

% {<init>}{<endinit>}{<citehook>}{<cite>}{<punct>}{<text>}{<apunct>}<tpunct>

defcsq@bquote@ii#1#2#3#4#5#6#7#8{csq@tquote@i{}{}{csq@oqopen@i}{#3}{#4}{#5}{#1#6#2}{#7}{#8}}

% {<init>}{<endinit>}{<citehook>}{<cite>}{<punct>}{<text>}{<apunct>}<tpunct>

longdefcsq@bquote@iii#1#2#3#4#5#6#7#8{%
  begin{csq@blockenvironment}%
  toggletrue{csq@block}%
  csq@setsfcodes
  edefcsq@tempa{%
    unexpanded{%
      mkblockquote
      {#1#6}%
      {#5}{#7}}%
    {#2ifblank{#4}
       {}
       {unexpanded{csq@switchlang{#3{#4}}}}}}%
  csq@tempa#8%
  end{csq@blockenvironment}}

renewrobustcmd*{foreignblockquote}[1]{%
  csq@getcargs{csq@bquote{csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}}}

renewrobustcmd*{hyphenblockquote}[1]{%
  csq@getcargs{csq@bquote{csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}}}

renewrobustcmd*{hybridblockquote}[1]{%
  csq@getcargs{%
    csq@bquote
      {iftoggle{csq@block}{csq@lang}{csq@hyph}{#1}tcsq@saveandreset@qlevel}
      {tcsq@restore@qleveliftoggle{csq@block}{csq@endlang}{csq@endhyph}}}}

renewrobustcmd*{foreignblockcquote}[1]{%
  csq@getccargs{csq@bquote{csq@lang{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endlang}}}

renewrobustcmd*{hyphenblockcquote}[1]{%
  csq@getccargs{csq@bquote{csq@hyph{#1}tcsq@saveandreset@qlevel}{tcsq@restore@qlevelcsq@endhyph}}}

renewrobustcmd*{hybridblockcquote}[1]{%
  csq@getccargs{%
    csq@bquote
      {iftoggle{csq@block}{csq@lang}{csq@hyph}{#1}tcsq@saveandreset@qlevel}
      {tcsq@restore@qleveliftoggle{csq@block}{csq@endlang}{csq@endhyph}}}}
makeatother

SetCiteCommand{autocite}
usepackage{lipsum}

begin{document}
  Lorem foreignquote{french}{Ceci est en français.}
  
  Lorem foreignquote{french}{Ceci enquote{enquote{est} en} français.}
  
  Lorem foreigntextquote{french}{Ceci enquote{enquote{est} en} français.}
  
  Lorem foreignblockcquote{french}{sigfridsson}{Ceci est en enquote{français}.}
  
  Lorem foreignblockcquote{french}{sigfridsson}{Ceci est en enquote{français}. lipsum}
end{document}