Foreign-Language support in part, Section, etc

I have a document with a lot of different languages, some of which appear in part{}, section{}, etc. blocks. For example, the following MWE:

documentclass[12pt,a4paper,oneside]{scrbook}
setkomafont{sectioning}{rmfamily bfseries}
usepackage{fontspec}
setmainfont[Mapping=tex-text,Numbers=OldStyle]{Linux Libertine O}
makeatletter
usepackage{csquotes}
usepackage{luacode}
usepackage{polyglossia}
setdefaultlanguage[variant=american]{english}
setotherlanguage[variant=poly]{greek}
usepackage{xstring}
defaultfontfeatures{Mapping=tex-text}
newfontfamilygreekfont[Script=Greek]{SBL Greek}
tracinglostchars=2

makeatother

begin{document}

part{Analysis Part Six: Anestis Logothetis{textemdash}{begin{greek}textsc{Άναστάσεις}end{greek}}}

Some text about {begin{greek}textsc{Άναστάσεις}end{greek}}

end{document}

For me, the word "Άναστάσεις" is properly set by textsc in the body text, but not in the title. If I try language setting in other languages (I have another section title with German) the "Part" name is preceded by "Titlegerman" (indicating that the "end{german} part just got appended to "Title").

Short version of the question: is there a way to have foreign-language words properly typeset in part, chapter, section, etc. blocks? And to appear this way in the TOC?

Additionally, I should point out that Noto Sans doesn’t handle textsc correctly at all for Greek scripts, which is why I’m sticking with SBL Greek here.

Thanks for the help!