The full stop in messages such as "$1. century" is useful in German, because its spelling rules mandate a full stop after numbers in some cases, but it is probably not needed in English.
Because of Wikibase's nonstandard date handling, some dates labelled "20. century" may actually be interpreted as a time in the period 2000–2099 by Wikibase components other than the user interface, and by software other than Wikibase.