LuaLaTeX requires euenc module in texlive-xetex. Without it \usepackege{polyglossia} throws LaTeX Error: Encoding scheme `EU2' unknown.
$ grep RDEPEND texlive-luatex/texlive-luatex-2015.ebuild RDEPEND="${DEPEND} dev-texlive/texlive-xetex seems ok to me
I've added euenc to TEXLIVE_MODULE_CONTENTS in my local texlive-xetex ebuild. Texlive-luatex package needs this change to work properly. This module was included in earlier versions of texlive-xetex ebuild.
euenc is in tl-latexrecommended in tl 2015 however, we have the following (R)DEPEND chain: tl-luatex -> tl-xetex -> tl-latexextra -> tl-latexrecommended so it should be fine and, actually, adding euenc to TEXLIVE_MODULE_CONTENTS in texlive-xetex should trigger package collisions
You're right, it's my mistake. My latexrecommended got downgraded. Sorry for the noise.