JSFiddle - React, Tailwind, and code Playground

HTML

<body class="pl">
    <div class="Standard">
        <div class="box-wymiar-pl">
            <p class="box" id="pl">Wybierz język</p>
            <p class="box" id="en">Choose language</p>
            <form class="align">
                <label class="sel_lang" for="pol_but">
                    <input id="pol_but" type="radio" name="sel_lang" value="polish" checked />
                    <img src="http://s21.postimg.org/bh3yqiw7n/Polish_Flag_ico.png">
                </label>
                <label class="sel_lang" for="eng_but">
                    <input id="eng_but" type="radio" name="sel_lang" value="english" />
                    <img src="http://s21.postimg.org/vyox268b7/English_Flag_ico.png" />
                </label>
                <br />
                <input type="submit" class="przycisk" value="" />
                </label>
            </form>
        </div>
    </div>
</body>

</html>

CSS

body.pl {
    background-color: black;
    background-image: url("http://s21.postimg.org/vr1a5nxcn/Polish_Flag.jpg");
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center center;
    margin: 0;
    min-width: 1224px;
    min-height: 866px;
    width: 100%;
    height: 866px;
}
body.en {
    background-color: black;
    background-image: url("http://s21.postimg.org/z9da21y8n/English_Flag.jpg");
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center center;
    margin: 0;
    min-width: 1224px;
    min-height: 866px;
    width: 100%;
    height: 866px;
}
div.Standard {
    font-family: Caviar Dreams;
    font-size: 24px;
}
div.box-wymiar-pl {
    background-image: url("http://s21.postimg.org/dx5s4deaf/bg_lang.jpg");
    background-repeat: repeat-x;
    border-radius: 5px;
    position: absolute;
    left: 50%;
    top: 50%;
    margin-top: -96.5px;
    margin-left: -201px;
    width: 100%;
    height: 100%;
    max-width: 402px;
    max-height: 193px;
    border-color: #000000;
    border:1px solid;
}
p.box#en {
    position: relative;
    margin: 0;
    display: none;
    margin-bottom: 5px;
    padding: 6px;
    text-align: center;
    text-shadow: 2px 2px 3px #cadc76;
}
p.box#pl {
    position: relative;
    margin: 0;
    margin-bottom: 5px;
    padding: 6px;
    text-align: center;
    text-shadow: 2px 2px 3px #cadc76;
}
.sel_lang > input[type=radio] {
    display:none;
}
input[type=radio] + img {
    cursor:pointer;
}
.align {
    text-align:center;
}
input[type=radio]:hover + img {
    box-shadow: 0 0 2px 1px lime;
}
input[type=radio]:checked + img {
    box-shadow: 0 0 2px 1px white;
}
.przycisk {
    margin-top: 5px;
    background-image: url('http://s21.postimg.org/l96mw674j/button.png');
    background-position: left, top;
    background-repeat: no-repeat;
    background-color: transparent;
    width: 114px;
    height: 42px;
    border: 0;
    cursor: pointer;
  ...

JavaScript

$(document).ready(

function () {
    $('#pol_but').click(

    function () {
        
        $('#en').hide();
        $('#pl').stop(true,true).fadeIn(2000);

        $('body').toggleClass('en pl');
    });
    $('#eng_but').click(

    function () {
        $('#pl').hide();
        $('#en').stop(true,true).fadeIn(2000);
        
        $('body').toggleClass('en pl');

    });
});