I worked for a French company, and I was forced to use web Excel in French, including keyboard shortcuts being FR-specific ("ctrl-g" for "bold" - "grossir").
I had my language set to English everywhere in browser, OS, MS profile, Office profile etc., not important.
(FWIW native Windows Excel didn't have this issue, so I sticked with native Excel).
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Re: location based on geoloc: I remember some HN user from Switzerland saying that IIRC either his IP is being moved around in geoIP databases between Swiss regions, or getting slightly distinct IP each time; as a result, he randomly gets all kinds of websites in French, and sometimes in German. Imagine this.