You get two warnings:
LaTeX Font Warning: Font shape `T2A/ntxtlf/m/n' undefined
(Font) using `T2A/cmr/m/n' instead on input line 12.
LaTeX Font Warning: Font shape `T2A/ntxtlf/bx/n' undefined
(Font) using `T2A/ntxtlf/m/n' instead on input line 12.
because newtxtext
doesn't support Cyrillic. Thus the standard font family is substituted (first warning), but the second substitution points back to the same font, because LaTeX knows nothing about ntxtlf
in T2A encoding.
Solution: substitute with a font family using Times, but supporting Cyrillic.
\documentclass[12pt]{article}
\usepackage[T2A,T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian,english]{babel}
%\usepackage{newtxtext}
\usepackage{tempora} % this supports Cyrillic
\usepackage{newtxmath}
\begin{document}
English: hello \textbf{hello}
Russian: \foreignlanguage{russian}{привет \textbf{привет}}
\end{document}
Note that, in any case, newtxmath
must be loaded after the font package for text, be it tempora
or newtxtext
.

You can use newtxtext
, though, provided you set up Tempora for Cyrillic.
\documentclass[12pt]{article}
\usepackage[T2A,T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian,english]{babel}
\usepackage{newtxtext}
\usepackage{newtxmath}
\usepackage{substitutefont}
\substitutefont{T2A}{\rmdefault}{Tempora-TLF}
\begin{document}
English: hello \textbf{hello}
Russian: \foreignlanguage{russian}{привет \textbf{привет}}
\end{document}
\documentclass[12pt]{article} \usepackage[T1,T2A]{fontenc} \usepackage[utf8]{inputenc} \usepackage[russian,english]{babel} \begin{document} hello \textbf{hello} \selectlanguage{russian} привет \textbf{привет} \end{document}
; modulo line breaks) works for me. Please show us a small example document that shows what you are trying to do, a so-called MWE tex.meta.stackexchange.com/q/228/35864.newtxtext
was certainly crucial in understanding the issue.