multibyte encoding characters handling by Javascript

My Colleague keep remind me not to use too advance Javascript which includes Ajax for building my webpages because he bumped into problems before when using Javascript to deal with multibyte characters. Because of that, I need to drop the whole concept of building a more interactive/ajax type of webpages. Currently, I am using quite a number of inner frames to minimized the reloading of the entire webpage whenever a certain part of the webpage needed to be changed. From my understanding, javascript usage can be as following examples :

1. javascript that used in between the server and client browser :
1a. User Keyed-in (data) on the HTML Form -> Javascript -> submit (data) to Server through normal form submission or Ajax
1b. Server response (data) to browser -> Javascript -> dynamically update the webpage with (data) probably on form elements/innerHTML/etc.

2. Javascript that used in between the User Interaction and client browser.
2a. User clicked on Add button -> Javascript -> new row of table containing selectbox with (data), text fields and labels(innerHTML) are dynamically created.

My concern is whether javascript is able to handle without any problem when all the (data) mentioned above is in all kinds of multibyte encoding characters? Will there be any corrupt of (data) when Javascript interact with it which may caused display of corrupted (data) on browser or submission of corrupted data to server?

I had tested before using multibyte encoding characters on Javascript and also Ajax, however, the characters I was trying are just a very small portion of the whole Universe of multibyte encoding characters. Because of that, the tests did not convinced my Colleague, saying that there will somehow be certain multibyte characters that cannot be handled by javascript (which I yet to see one). If what my colleague said is true, I will be in big trouble if I already started developing my interactive webpages with alot of advance Javascript and Ajax.

So I hope someone could confirm with me that currently Javascript can be used to handle any kind of multibyte encoding characters, and I should not concern anymore about it because this problem only exist few years back in the past. Thanks.

looks like is a hard question from me, hopefully someone out there could give me some hints or suggestion on how to find the answer or how to proceed. Thanks.