Wrong alignment in parallel text with selectlanguage

The problem is selectlanguage{greek} that adds a blank line. Use begin{otherlanguage*}{greek}...end{otherlanguage*} instead.

I propose a simpler interface:

documentclass[11pt,a4paper]{report}
usepackage[left=3cm, right=3cm, top=3cm, bottom=3cm, headheight=13.6pt]{geometry}
usepackage[T1]{fontenc}
usepackage{gfsdidot}
usepackage[main=english, greek, french]{babel}
languageattribute{greek}{ancient}
usepackage{parallel}
%usepackage{xparse} % not needed with LaTeX 2020-10-01 or later

NewDocumentEnvironment{xParallel}{mmo}
 {%
  IfValueT{#3}{%
    renewcommand{ParallelAtEnd}{begin{flushright}#3end{flushright}}%
  }%
  Parallel{#1}{#2}%
 }
 {endParallel}

NewDocumentCommand{ParallelText}{mom}{%
  csname Parallel#1Textendcsname{%
    IfValueT{#2}{begin{otherlanguage*}{#2}}%
    #3par
    IfValueT{#2}{end{otherlanguage*}}%
  }%
}

begin{document}

begin{xParallel}{0.52textwidth}{0.43textwidth}[
  textit{Top.} I 1, 100a25--b23; transl. Smith modified
]
ParallelText{L}{
  A textit{deduction}, then, is a discourse in which,
  certain things having been supposed, something different
  from the things supposed results of necessity through them. 
}
ParallelText{R}[greek]{raggedright
   Ἔστι δὴ συλλογισμὸς λόγος ἐν ᾧ τεθέντων τινῶν ἕτερόν τι τῶν 
  κειμένων ἐξ ἀνάγκης συμβαίνει διὰ τῶν κειμένων.
}
end{xParallel}

end{document}

The xParallel environment has a trailing optional argument for setting the final text. The ParallelText command has a mandatory argument for the side (L or R) and also an optional argument for the language.

The advantage is that it's possible to fine tune the spacings. For instance, you may want to do

documentclass[11pt,a4paper]{report}
usepackage[left=3cm, right=3cm, top=3cm, bottom=3cm, headheight=13.6pt]{geometry}
usepackage[T1]{fontenc}
usepackage{gfsdidot}
usepackage[main=english, greek, french]{babel}
languageattribute{greek}{ancient}
usepackage{parallel}
%usepackage{xparse} % not needed with LaTeX 2020-10-01 or later

usepackage{lipsum}

NewDocumentEnvironment{xParallel}{mmo}
 {%
  paraddvspace{topsep}
  IfValueT{#3}{%
    renewcommand{ParallelAtEnd}{parnobreakvspace{topsep}{raggedleft#3par}}%
  }%
  Parallel{#1}{#2}%
 }
 {endParallelparaddvspace{topsep}}

NewDocumentCommand{ParallelText}{mom}{%
  csname Parallel#1Textendcsname{%
    IfValueT{#2}{begin{otherlanguage*}{#2}}%
    #3par
    IfValueT{#2}{end{otherlanguage*}}%
  }%
}

begin{document}

lipsum[1][1-3]

begin{xParallel}{0.52textwidth}{0.43textwidth}[
  textit{Top.} I 1, 100a25--b23; transl. Smith modified
]
ParallelText{L}{
  A textit{deduction}, then, is a discourse in which,
  certain things having been supposed, something different
  from the things supposed results of necessity through them. 
}
ParallelText{R}[greek]{raggedright
   Ἔστι δὴ συλλογισμὸς λόγος ἐν ᾧ τεθέντων τινῶν ἕτερόν τι τῶν 
  κειμένων ἐξ ἀνάγκης συμβαίνει διὰ τῶν κειμένων.
}
end{xParallel}

lipsum[1][1-3]

end{document}