General questions regarding language setup on JCOs

Hello,

When selecting a language in the JCO configuration, for example English (UK) or English (US) in the SAP_R3_HumanResources JCO, where will I see the effect of this? Will that affect for example the date format in the selection boxes of ESS/MSS? If yes then does that override the date format of the user's master record on ECC? Or the portal master configuration?

Roy