What language is the song "Fallen Heroes" in Akame ga Kill?
The vocalist on "Le chant de Roma" is Maïa Barouh. After dredging through her various social media accounts, I found the following exchange on her Facebook page:


  Antoine Prt Maïa Barouh, quelle est la langue utilisée dans ce morceau ? :)
  
  
    Maïa Barouh Du gromeleu :p une langue inventée
  


"langue inventée" - that is, an invented language. Straight from the horse's mouth.

(Of course, the piece's lyricist certainly would have taken inspiration from various real languages, but I think it's fair to say that the text of "Le chant de Roma" is not, itself, in any real language.)