Texlive 2020. No hyphenation patterns were preloaded for `Russian`.

charlzk@localhost ~/tmp/rc4 $ sudo /usr/sbin/texmf-update 
Пароль: 
Configuring TeXLive ...
Generating /etc/texmf/web2c/texmf.cnf from /etc/texmf/texmf.d ...
Generating /etc/texmf/web2c/fmtutil.cnf from /etc/texmf/fmtutil.d ...
cat: '/etc/texmf/fmtutil.d/*.cnf': Нет такого файла или каталога
Generating /etc/texmf/web2c/updmap.cfg from /etc/texmf/updmap.d ...
Generating ls-R files
Generating language.dat file
Generating language.def file
Generating language.dat.lua file
Generating format files ...
Skipping font maps, since they are unchanged

Use 'texconfig font ro'(rw) to disable (enable) font generation for users

\usepackage{hyphsubst}

charlzk@localhost ~/tmp/rc4 $ cat /usr/share/texmf-dist/tex/generic/config/language.def 
%% e-TeX V2.0;2
% Note: the first line of this file must match the corresponding line
% in "etex.src" and "etexdefs.lib", otherwise fallback will be used.

% $Id: language.us.def 18737 2010-06-04 17:09:02Z karl $
% language.us.def (and the start of language.def)
% - initial hyphenation patterns.

% DO NOT EDIT THIS FILE (language.def)! It is generated by tlmgr.
% See language.dat (or language.us) for more information.

% It is explicitly permitted to augment this file by adding additional
% \addlanguage ... commands AFTER the first (\addlanguage {USenglish})
% which must remain as the first language added to ensure total document
% portability for pre-\language documents.  The last line of the file
% MUST be left as \uselanguage {USenglish} for the sane reason.

% Additional languages, patterns, exceptions and left- and right-hyphen
% minima may be added here, and an example is given for German.

% The five parameters for \addlanguage are:
% language, patterns file, exceptions file, left- and right-hyphen minima

%%% Next section is      E X A M P L E   O N L Y
%%% \addlanguage {German}{ghyph31}{}{2}{2}   %%% further such lines may be used;
%%% Previous section is  E X A M P L E   O N L Y

\addlanguage {USenglish}{hyphen}{}{2}{3} %%% This MUST be the first non-comment
                                         %%% line of the file

\addlanguage{belarusian}{loadhyph-be.tex}{}{2}{2}
\addlanguage{bulgarian}{loadhyph-bg.tex}{}{2}{2}
\addlanguage{churchslavonic}{loadhyph-cu.tex}{}{1}{2}
\addlanguage{mongolian}{loadhyph-mn-cyrl.tex}{}{2}{2}
\addlanguage{mongolianlmc}{loadhyph-mn-cyrl-x-lmc.tex}{}{2}{2}
\addlanguage{russian}{loadhyph-ru.tex}{}{2}{2}
\addlanguage{serbian}{loadhyph-sr-latn.tex}{}{2}{2}
\addlanguage{serbianc}{loadhyph-sr-cyrl.tex}{}{2}{2}
\addlanguage{ukrainian}{loadhyph-uk.tex}{}{2}{2}
%%% No changes may be made beyond this point.

\uselanguage {USenglish}             %%% This MUST be the last line of the file.

charlzk@localhost ~/tmp/rc4 $ cat /usr/share/texmf-dist/tex/generic/config/language.dat
% $Id: language.us 18738 2010-06-04 17:18:14Z karl $
% language.us (and the start of language.dat)
% - initial hyphenation patterns.
% Created long ago by Sebastian Rahtz and others.  Public domain.
%
% DO NOT EDIT THIS FILE (language.dat)! It is generated by tlmgr.
%
% The premade texmf/tex/generic/config/language.dat* files (where you
% might be reading this) in the distribution was generated with all
% languages enabled.  It is used when running live.
%
% To enable or disable languages, there are two options:
% - if the language is provided by a TeX Live package, you can
%   (un)install the corresponding package using tlmgr. Examples:
%     tlmgr install hyphen-french
%     tlmgr remove hyphen-english
% - in any case, you can use local configuration files:
%     TEXMFLOCAL/tex/generic/config/language-local.dat
%     TEXMFLOCAL/tex/generic/config/language-local.dat.lua
%     TEXMFLOCAL/tex/generic/config/language-local.def
%   See the tlmgr documentation (try `man tlmgr' or `texdoc tlmgr'),
%   section "generate" for details.
%
% The preferred comment for actual language lines in language.dat and
% language.us is %!. Do not use %!  for textual comments like this. This
% is to be able to separate textual comments from commented-out language
% in such a way that automatic parsing by programs is possible, so that
% we can present the user with `all possible languages'.
% 
% We must keep english as the default (first) here, and let it refer to
% hyphen.tex (not anything else), and do not change the hyphen.tex file,
% or name some other file hyphen.tex.  In other words, hyphen.tex must
% remain the original file from Knuth, and it must be \language0.  This
% is one important aspect of ensuring that the original, frozen TeX
% always produces the same line and page breaks.
% 
% The babel system allows you to easily change the active language for
% LaTeX. For more information, see the documentation in
% texmf-dist/doc/generic/babel/.  ConTeXt has its own language support too.
% 
english		hyphen.tex  % do not change!
=usenglish
=USenglish
=american
%
% ushyphmax.tex, on the other hand, includes Gerard Kuiken's additional
% patterns; it is not frozen.  It is provided by the hyphen-english
% package, not listed here.  Also, in hyph-utf8 and current TL, it has
% been renamed to hyph-en-us.tex, along with other *hyph-en-us.* files.
%
% FYI, ushyph.tex is Dr. Kuiken's smaller set of patterns; with today's
% large memories, there is no reason to use it, and we don't list it here.
% ushyph1.tex is another (historical) name for hyphen.tex.
% ushyph2.tex is another (historical) name for ushyph.tex.
% --karl
% 
dumylang	dumyhyph.tex    %for testing a new language.
nohyphenation	zerohyph.tex    %a language with no patterns at all.
belarusian loadhyph-be.tex
bulgarian loadhyph-bg.tex
churchslavonic loadhyph-cu.tex
mongolian loadhyph-mn-cyrl.tex
mongolianlmc loadhyph-mn-cyrl-x-lmc.tex
russian loadhyph-ru.tex
serbian loadhyph-sr-latn.tex
serbianc loadhyph-sr-cyrl.tex
ukrainian loadhyph-uk.tex

\documentclass[10pt]{article}
\usepackage{hyphsubst}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian]{babel}
\usepackage{amsmath}
\title{RC4}

Package babel Warning: No hyphenation patterns were preloaded for
(babel)                the language `Ukrainian' into the format.
(babel)                Please, configure your TeX system to add them and
(babel)                rebuild the format. Now I will use the patterns
(babel)                preloaded for english instead on input line 31.

) (/usr/share/texmf-dist/tex/generic/babel-russian/russianb.ldf

Package babel Warning: No hyphenation patterns were preloaded for
(babel)                the language `Russian' into the format.
(babel)                Please, configure your TeX system to add them and
(babel)                rebuild the format. Now I will use the patterns
(babel)                preloaded for english instead on input line 28.

fmtutil-sys --all

charlzk@localhost ~ $ sudo fmtutil-sys --all
Пароль: 
fmtutil: fmtutil is using the following fmtutil.cnf files (in precedence order):
fmtutil:   /etc/texmf/web2c/fmtutil.cnf
fmtutil:   /usr/share/texmf-dist/web2c/fmtutil.cnf
fmtutil: fmtutil is using the following fmtutil.cnf file for writing changes:
fmtutil:   //etc/texmf/web2c/fmtutil.cnf
fmtutil [INFO]: writing formats under /var/lib/texmf/web2c
Use of uninitialized value $whatarg in concatenation (.) or string at /usr/bin/fmtutil line 443.
fmtutil [INFO]: did not find entry for all=, skipped
fmtutil [INFO]: total formats: 0
fmtutil [INFO]: exiting with status 0

sudo emerge --ask app-text/texlive

charlzk@localhost ~ $ cat /etc/portage/make.conf
# These settings were set by the catalyst build script that automatically
# built this stage.
# Please consult /usr/share/portage/config/make.conf.example for a more
# detailed example.
CFLAGS="-O2 -pipe"

# NOTE: This stage was built with the bindist Use flag enabled
PORTDIR="/usr/portage"
DISTDIR="/usr/portage/distfiles"
PKGDIR="/usr/portage/packages"

# This sets the language of build output to English.
# Please keep this setting intact when reporting bugs.
LC_MESSAGES=C
GRUB_PLATFORMS="efi-64"
INPUT_DEVICES="evdev synaptics"
VIDEO_CARDS="radeon amdgpu radeonsi"

# Logs for Portage
PORTAGE_ELOG_CLASSES="log warn info error"
PORTAGE_ELOG_SYSTEM="save"
USE="pulseaudio alsa"
source /var/lib/layman/make.conf

# Enabling ccache
FEATURES="${FEATURES} ccache"
CCACHE_DIR="/gentoo/ccache"

# Licenses
ACCEPT_LICENSE="*"

# Languages
L10N="en ru"

charlzk@localhost ~ $ sudo emerge --ask --update --newuse --changed-use --deep @world

\documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T2A]{fontenc}
\usepackage[english,russian]{babel}
\usepackage{inputfirst}
\frenchspacing

\begin{document}
 Проверка testing 123
\end{document}

charlzk@localhost ~/tmp/minimal $ cat minimal.tex 
\documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T2A]{fontenc}
\usepackage[english,russian]{babel}
\usepackage{inputfirst}
\frenchspacing

\begin{document}
 Проверка testing 123
\end{document}

charlzk@localhost ~/tmp/minimal $ pdflatex minimal.tex 
This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020 Gentoo Linux) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./minimal.tex
LaTeX2e <2020-02-02> patch level 5
L3 programming layer <2020-02-25>
(/usr/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(/usr/share/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texmf-dist/tex/latex/base/inputenc.sty)
(/usr/share/texmf-dist/tex/latex/base/fontenc.sty
(/usr/share/texmf-dist/tex/latex/cyrillic/t2aenc.def
(/usr/share/texmf-dist/tex/latex/base/t2aenc.dfu))
(/usr/share/texmf-dist/tex/latex/cyrillic/t2acmr.fd))
(/usr/share/texmf-dist/tex/generic/babel/babel.sty
(/usr/share/texmf-dist/tex/generic/babel/switch.def)
(/usr/share/texmf-dist/tex/generic/babel-english/english.ldf
(/usr/share/texmf-dist/tex/generic/babel/babel.def
(/usr/share/texmf-dist/tex/generic/babel/txtbabel.def)))
(/usr/share/texmf-dist/tex/generic/babel-russian/russianb.ldf

Package babel Warning: No hyphenation patterns were preloaded for
(babel)                the language `Russian' into the format.
(babel)                Please, configure your TeX system to add them and
(babel)                rebuild the format. Now I will use the patterns
(babel)                preloaded for english instead on input line 28.

))

! LaTeX Error: File `inputfirst.sty' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)

Enter file name: