Hey,
These language keys are in the en-GB.plg_sellaciouspayment_paypalstandard.ini while this plugin is only taking language from the en-GB.plg_sellaciouspayment_paypalstandard.sys.ini in the folder plugins/sellaciouspayment/paypalstandard/language/en-GB.
Noted, and we will fix this issue. For now, you can copy the missing language keys from the first file to the second file and it will work.
I hope this helps.
Thank You