wemon,
добавить на обе страницы.
<!DOCTYPE html>
<html >
<head ></head>
<body><select class="lang">
<option value="ru">ru</option>
<option value="ua">ua</option>
</select>
</body>
<script>
document.addEventListener("DOMContentLoaded", () => {
const select = document.querySelector(".lang");
let url = window.location.href, value;
for ({
value
} of select.options) {
if (value && url.includes(value, 19)) {
select.value = value;
}
};
select.addEventListener("change", _ => {
if(!select.value) return;
url = `https://stock.ck.ua/${select.value}/catalog/7451362/`;
window.location.href = url;
})
});
</script>
</html>