If you want your wiki to show up correctly in Opera there is a possible change in the styles. You need first to create a skin (See How To Create ASkin)

The style to add is:

.xwikiactions, #xwikiwebusermenu, #xwikilanguagemenu {
    display: block;
    float: left;
    margin: 0px;
    padding: 0px;
    height: 1em;
    width: auto;
    /* Tantek hack pour cacher les 29.9ex à IE5 : */
    voice-family: ""}"";
    voice-family:inherit;
    width: 40%;
    }
html*#xwikilanguagemenu {
    width: auto;
}

The result looks ok in Opera but makes Internet Explorer and Firefox not show the menu perfectly aligned on the right. This is the reason why we didn't add the fix in the standard style sheet.