Edit in JSFiddle

<button class="submit-btn">Kliknij</button>
<button class="submit-btn">te</button>
<button class="submit-btn">przyciski</button>

.submit-btn {
    font-family: sans-serif;
    padding:1rem 2rem;
    background: #70B81B;
    border:0;
    border-radius:0.2rem;
    color:#fff;
    font-size:1.1rem;
    font-weight: bold;
    transition: 0.3s background-color;
}

.element-is-busy {
    position: relative;
    pointer-events: none;
    opacity:0.5;
}
.element-is-busy::after {
    position: absolute;
    left: 50%;
    top: 50%;
    width: 20px;
    height: 20px;
    border-radius: 50%;
    border: 2px solid rgba(0, 0, 0, 0.2);
    border-right-color: rgba(0,0,0,0.7);
    transform: translate(-50%, -50%) rotate(0deg);
    content:'';
    animation: rotateSingleLoading 0.3s infinite linear;
    z-index: 100;
}
@keyframes rotateSingleLoading {
    from {
        transform: translate(-50%, -50%) rotate(0deg);
    }
    to {
        transform: translate(-50%, -50%) rotate(360deg);
    }
}