index.html

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8"> 
<link rel="stylesheet" href="formvalidation.css">
<style type="text/css">
body, select, input {
    font-size: 14px;
}

.change-lang-btn:after { 
	content: 'ru';
	font-weight: bold;
}

.change-lang-btn:lang(ru):after { 
	content: 'en' 
}
</style>
</head>
<body>
    <button class="change-lang-btn" onclick="document.documentElement.lang = document.documentElement.lang === 'en' ? 'ru' : 'en'">
        Change plugin language to 
    </button>
    <form action="">
        <p>
            <label>
            Your name
            <input type="text" required name="name">
            </label>
        </p>
        <p>
            <label>
            Your email
            <input type="email" required name="email">
            </label>
        </p>
        <p>
            <label>
            Your gender
            <select name="gender">
                <option>male</option>
                <option>female</option>
            </select>
            </label>
        </p>
        <p>
            <label>
            Your birthdate
            <input type="date" name="birthdate" required>
            </label>
        </p>
        <p>
            <input type="checkbox" name="accept" required/>
            <label>I accept site rules</label>
        </p>
        <button type="submit">Submit</button>
    </form>
    <script src="formvalidation.js"></script>
</body>
</html>