function checkForm(lang) {
	err = false;
	base = new Array();
	base[0] = 'Vyplňte prosím všetky povinné údaje označené hviezdičkou (*).';
	base[1] = 'Please, fill all required fields marked with asterix (*).';

	// empty fields
	if(document.getElementById('contact_company').value == '') err = true; // empty company VAL
	if(document.getElementById('contact_address').value == '') err = true; // empty address VAL
	if(document.getElementById('contact_person').value == '') err = true; // empty person VAL
	if(document.getElementById('contact_phone').value == '') err = true; // empty phone VAL
	if(document.getElementById('contact_email').value == '') err = true; // empty mail VAL
	if(document.getElementById('contact_location').value == '') err = true; // empty location VAL
	if(document.getElementById('tech_1_A').value == '' && document.getElementById('tech_1_B').value == '') err = true; // empty length VAL
	if(document.getElementById('tech_2').value == '') err = true;
	if(document.getElementById('tech_3').value == '') err = true;

	// radio checks, only required
	if(document.getElementById('type_1').checked == false && document.getElementById('type_2').checked == false && document.getElementById('type_3').checked == false) err = true;
	if((document.getElementById('tech_4_1_A').checked == false && document.getElementById('tech_4_1_B').checked == false) || (document.getElementById('tech_4_2_A').checked == false && document.getElementById('tech_4_2_B').checked == false)) err = true;
	if(document.getElementById('tech_5_A').checked == false && document.getElementById('tech_5_B').checked == false && document.getElementById('tech_5_C').checked == false && document.getElementById('tech_5_D').checked == false) err = true;

	// results
	if(err == true) {
		alert (base[lang]);
		return false;
	}
	else return true;

}

function changeSeats(what) {
	if(what == 1) {
		document.getElementById('tech_5_A').disabled = false;
		document.getElementById('tech_5_B').disabled = true;
		document.getElementById('tech_5_C').disabled = true;
		document.getElementById('tech_5_D').disabled = true;
	}
	else {
		document.getElementById('tech_5_A').disabled = true;
		document.getElementById('tech_5_B').disabled = false;
		document.getElementById('tech_5_C').disabled = false;
		document.getElementById('tech_5_D').disabled = false;
	}
}