http://rockingsoccer.com/en/soccer/friendlies/friendlies
First of all, it says that I can play 0.5 friendliest a day. Does that mean 1 every 2 days?
Secondly, what does credits have to do with anything? I checked my real credits and it hasn't gone down.

First question - yes.

Second - those are different credits, only name is the same; although the name is already changed (to friendly-credits or similar) in other translations, but looks like english version still uses the old one.

I understand that about credits, but what are they used for and when are they reset?

Those friendly-credits are only used for friendlies.

Ain't sure what's their recovery rate since I barely play any friendlies, so you will need to wait till Vincent shows up or some other user who plays friendlies more often.

You recover 0.5 friendly credit per day, but you see it rounded to the full digit. The maximum amount of friendly credits is 5. You can play the friendly game only with a user who has more than one friendly credit.

Everything is explained here:
http://rockingsoccer.com/en/football/friendlies/friendlies

I advise to read the tips on every page. They are pretty much self-explanatory.