Simulate a click using javascript once page is loaded

simply adding this:

$('.translate').trigger('click');

Select allOpen in new window

Note if you have more than one button on the page with that class your code will click all of them.

If that is not the required action you will need to isolate the button either with a parent element or with an id.

Yes, so in the code I posted you can see that there is an id check.
and so, yeah, how do I do that?
(isolate the button).
and, again, here is the code snippet of the part that 'watches' for one of the 'languages' button press.

var lang = $(this).attr('id');

Select allOpen in new window

and it's probably preferred that this is done after page load.  Does that mean putting it at the end of the page?

and it's probably preferred that this is done after page load.  Does that mean putting it at the end of the page?

you can do it like this:

$(document).ready(function(){
      $('.translate').click(function() {
        var lang = $(this).attr('id');
        alert(lang);
      });

      $('.translate').trigger('click');
});

Select allOpen in new window

again, here is the code snippet of the part that 'watches' for one of the 'languages' button press.

are the buttons come with same id or different ids?

I'm not too sure why you accepting your own comment as the answer.

var lang = 'jp';
which didn't work.

but you didn't share your HTML element codes so we got no chance to debug for you.

Question - on page load how do you know which button you want to click?

If you have a button with id = 'jp' then all you need to do is

document.getElementById('jp').click();

Select allOpen in new window

If you want to remember which button a user clicked you can use localStorage
HTML

<button id="jp" class="translate">JP</button>
<button id="en" class="translate">EN</button>
<button id="reset">Reset</button>

Select allOpen in new window

JavaScript

<script>
$(function() {
	$('#reset').click(function(e) {
		e.preventDefault();
		localStorage.removeItem('translateBtn');
	});
	
	$('.translate').click(function() {
		alert('Button ' + this.id + ' clicked!');
	   localStorage.setItem('translateBtn', this.id);
	});

	var btnId = localStorage.getItem('translateBtn');
	if (btnId) {
	   document.getElementById(btnId).click();
	}	
});
</script>

Select allOpen in new window

Working sample here

Sorry Ryan Chong, but your responses didn't address what was needed, despite me providing enough information.
Your last response was to use [ var lang = $(this).attr('id');] when there wouldn't be any 'this' on a page load.

Julian's solution came after I had just decided to duplicate the function and hardwire the language.  he guessed that JP would be the one I'd want to open the site with first.
However document.getElementById('jp').click(); isn't working, even when I put it at the end of the page.  I thought about trying body onload, but directly running a copy of the function loads up the language first and probably renders the page sooner.  It's ugly, but it works.

Thanks everyone for your efforts.

However document.getElementById('jp').click(); isn't working, even when I put it at the end of the page

Show us your full code so we can see the implementation.

The sample I provided demonstrates the concept correctly - if it is not working for you then it is down to implementation. If we can see what you have done then we can tell you why your implementation is not working.