No I don't think so. See - I'm currently not developing with extjs, just evaluating it if it could serve our purposes. So I'm just browsing the examples ON www.extjs.com and there are evident encoding bugs that strike me.
I do not know what to expect for "ukrainian month names", but I know when I see an encoding error. I gave you the URL to "your" website. If you do not see an encoding error there yourself, I might provide a screenshot.
I have my constructive day today... see attachment