My custom X11 keyboard layout with support for latin (polish, german, english) and cyrillic (russian, ukrainian, serbian, belarussian) writing, based on qwerty. To be put in /usr/share/xkeyboard-config-2/symbols

latin_cyrillic_custom

default partial alphanumeric_keys
xkb_symbols "basic" {
    include "latin"

    name[1] = "Latin";

    key <TLDE> { [ grave, asciitilde, doublelowquotemark, rightdoublequotemark ] };
    key <AE01> { [ 1, exclam, notequal, exclamdown ] };
    key <AE02> { [ 2, at, twosuperior, questiondown ] };
    key <AE04> { [ 4, dollar, EuroSign, sterling ] };
    key <AE05> { [ 5, percent, ellipsis, permille ] };
    key <AE06> { [ 6, asciicircum, onehalf, logicaland ] };
    key <AE07> { [ 7, ampersand, section, U2248 ] };
    key <AE08> { [ 8, asterisk, U221A, threequarters ] };
    key <AE09> { [ 9, parenleft, guillemotleft, plusminus ] };
    key <AE10> { [ 0, parenright, guillemotright, degree ] };
    key <AE11> { [ minus, underscore, endash, emdash ] };

    key <AD01> { [ q, Q, Greek_pi, Greek_OMEGA ] };
    key <AD03> { [ e, E, eogonek, Eogonek ] };
    key <AD04> { [ r, R, copyright, registered ] };
    key <AD05> { [ t, T, ssharp, trademark ] };
    key <AD06> { [ y, Y, adiaeresis, Adiaeresis ] };
    key <AD09> { [ o, O, oacute, Oacute ] };

    key <AC01> { [ a, A, aogonek, Aogonek ] };
    key <AC02> { [ s, S, sacute, Sacute ] };
    key <AC05> { [ g, G, odiaeresis, Odiaeresis ] };
    key <AC06> { [ h, H, udiaeresis, Udiaeresis ] };
    key <AC07> { [ j, J, schwa, SCHWA ] };

    key <AB01> { [ z, Z, zabovedot, Zabovedot ] };
    key <AB02> { [ x, X, zacute, Zacute ] };
    key <AB03> { [ c, C, cacute, Cacute ] };
    key <AB06> { [ n, N, nacute, Nacute ] };
    key <AB07> { [ m, M, mu, infinity ] };
    key <AB08> { [ comma, less, lessthanequal, multiply ] };
    key <AB09> { [ period, greater, greaterthanequal, division ] };

    key <SPCE> { [ space, space, nobreakspace, nobreakspace ] };

    include "level3(ralt_switch)"
};

partial alphanumeric_keys
xkb_symbols "cyrillic" {
    include "latin"

    name[2] = "Cyrillic";

    key <TLDE> { [ grave, asciitilde, guillemotleft, guillemotright ] };

    key <AD01> { [ Cyrillic_sha, Cyrillic_SHA, Cyrillic_shcha, Cyrillic_SHCHA ] };
    key <AD02> { [ Cyrillic_ve, Cyrillic_VE ] };
    key <AD03> { [ Cyrillic_ie, Cyrillic_IE, Cyrillic_e, Cyrillic_E ] };
    key <AD04> { [ Cyrillic_er, Cyrillic_ER, Ukrainian_ie, Ukrainian_IE ] };
    key <AD05> { [ Cyrillic_te, Cyrillic_TE, Serbian_tshe, Serbian_TSHE ] };
    key <AD06> { [ Cyrillic_yeru, Cyrillic_YERU, Ukrainian_yi, Ukrainian_YI ] };
    key <AD07> { [ Cyrillic_u, Cyrillic_U, Cyrillic_yu, Cyrillic_YU ] };
    key <AD08> { [ Cyrillic_i, Cyrillic_I, Ukrainian_i, Ukrainian_I ] };
    key <AD09> { [ Cyrillic_o, Cyrillic_O, Cyrillic_io, Cyrillic_IO ] };
    key <AD10> { [ Cyrillic_pe, Cyrillic_PE ] };

    key <AC01> { [ Cyrillic_a, Cyrillic_A, Cyrillic_ya, Cyrillic_YA ] };
    key <AC02> { [ Cyrillic_es, Cyrillic_ES, U046B, U046A ] };
    key <AC03> { [ Cyrillic_de, Cyrillic_DE, Serbian_dje, Serbian_DJE ] };
    key <AC04> { [ Cyrillic_ef, Cyrillic_EF ] };
    key <AC05> { [ Cyrillic_ghe, Cyrillic_GHE, Ukrainian_ghe_with_upturn, Ukrainian_GHE_WITH_UPTURN ] };
    key <AC06> { [ Cyrillic_ha, Cyrillic_HA, U0467, U0466 ] };
    key <AC07> { [ Cyrillic_shorti, Cyrillic_SHORTI, Cyrillic_je, Cyrillic_JE ] };
    key <AC08> { [ Cyrillic_ka, Cyrillic_KA, Byelorussian_shortu, Byelorussian_SHORTU ] };
    key <AC09> { [ Cyrillic_el, Cyrillic_EL, Cyrillic_lje, Cyrillic_LJE ] };

    key <AB01> { [ Cyrillic_ze, Cyrillic_ZE, U046D, U046C ] };
    key <AB02> { [ Cyrillic_softsign, Cyrillic_SOFTSIGN, Cyrillic_hardsign, Cyrillic_HARDSIGN ] };
    key <AB03> { [ Cyrillic_tse, Cyrillic_TSE, Cyrillic_che, Cyrillic_CHE ] };
    key <AB04> { [ Cyrillic_zhe, Cyrillic_ZHE, Cyrillic_dzhe, Cyrillic_DZHE ] };
    key <AB05> { [ Cyrillic_be, Cyrillic_BE, U0469, U0468 ] };
    key <AB06> { [ Cyrillic_en, Cyrillic_EN, Cyrillic_nje, Cyrillic_NJE ] };
    key <AB07> { [ Cyrillic_em, Cyrillic_EM ] };

    include "level3(ralt_switch)"
};

Phonetic cyrillic layout (the main feature):

Bottom right corner is with alt gr.