It had to be one of those fun declensions!!! I rely on 'Google Translate' for word translations. By now, I know that 'oni. one' are the main 'they' pronouns. I wonder why Google would show 'they, these' for 'ci'. That is, in Polish, is 'ci' ever implied 'they' as 'These' would be in English? Thanks.
Google Translate is quite good in getting the general meaning of the sentence - but for grammar is not the best choice. And not for words, that have several meanings. I would not recommend ;)
So, 'ci' is both short dative singular of 'ty' ('wierzę ci' = 'I believe you'), and personal masculine plural of 'ten' ('ci mężczyźni'='these men'). No idea why it translates as 'they'.