When uselang=xyz is specified in the URL RevisionSlider only considers it for the first diff shown. Once user selects other revisions custom language is ignored and diff for the language version selected in user preferences is loaded instead (this affects the langauage of reloaded parts of UI). Furthermore, new URLs generated by the extension do not contain uselang parameter.
See below how it looks like on wiki with default and user language set to English and URL initially containing uselang=he
- URL no longer includes uselang=he
- Diff headers are in English instead of Hebrew