The principle that costs follow the event is a common law principle that the unsuccessful party pays the legal costs of the successful party in litigation.
http://lawgradinpink.blogspot.com/2017/03/costs-in-cause-v-c...

An award of costs will generally flow with the result of litigation; the successful party being entitled to an order for costs against the unsuccessful party.
http://www.duhaime.org/LegalDictionary/C/CostsFollowTheEvent...

The general rule in costs jurisdiction is that "costs follow the event". In other words, the loser in legal proceedings must pay the legal costs of the successful party.
https://www.proz.com/kudoz/english-to-spanish/law-general/16...

II - Dá causa às custas do processo a parte vencida, na proporção em que o for. Fica vencido quem na causa não viu os seus interesses satisfeitos, total ou parcialmente.
http://www.pgdlisboa.pt/jurel/jur_mostra_doc.php?nid=5566&co...

É isso que se diz em pt-pt.

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Note added at 52 mins (2020-07-28 14:26:56 GMT)
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Encontra várias ocorrências para pt-pt:

https://www.google.pt/search?biw=2560&bih=1329&sxsrf=ALeKk00...

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Note added at 1 hr (2020-07-28 14:46:07 GMT)
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Outra forma de traduzir e que aparece também com frequência:

33 Under Article 69(2) of the Rules of Procedure, costs follow the event. I therefore propose that the defendants should be ordered to pay the costs.

33 Nos termos do artigo 69._, n._ 2, do Regulamento de processo, a parte vencida é condenada nas despesas. Proponho assim que as demandadas sejam condenadas nas despesas.

https://eur-lex.europa.eu/legal-content/EN-PT/TXT/?qid=15959...

"... codifies the 'loser-pays' or 'costs follow the event' rule."

Traduzia assim:

"...estipula que a parte vencida é condenada a pagar/no pagamento as/das custas processuais"

"Event" tem o sentido de "processo", "causa", "ação".