I cannot mix characters from different languages in one document. When I add \usepackage[T1]{fontenc}, there are problems with the cyrillic characters. When I add T2A, there are problems with other characters.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1,T2A]{fontenc}
\begin{document}
\section{Test}
aşçęÐпщ
\end{document}
T1:
Unicode character п (U+043F)
(inputenc) not set up for use with LaTeX.
T1,T2A:
Command \DH unavailable in encoding T2A.


xelatexandlualatexare perfectly stable and the best way to go – DG' Oct 13 '19 at 18:28