For starters, everything starts later here. So on Saturday and Sunday, there's just not going to be much until late afternoon at the earliest.
The Alamo (on Uruguay... google it) tends to have a sports seeking crowd on the weekends. It can be 3 people, it can be 15 people (plus Argentines).
I'm not sure what time Gibraltar opens up in San Telmo, but it definitely caters to an expat crowd. Pool, curry, draft beers with flavor... I believe that Bangalore in Palermo Hollywood is owned by the same people. Same feel, but probably fewer expats.
The Recoleta Cemetery area. You can hear English all over the place.
Sugar (on Costa Rica... google) is a later place. It opens at 8pm, but is usually pretty quiet until at least 10pm or later. There aren't any TVs there (televisions, in ~12 months of going there I think that I've seen one transvestite). There's good music and on Friday and Saturday night, it is open until 6am.
Milion (on Parana, again use The Google) is another place that caters to expats (mainly because of the price). It's not a pub. It's a bar & restaurant and the main part isn't open until evening. There is a cafe/sandwicheria downstairs that opens around 4pm, but is not a "hangout".
Shamrock (on Rodriguez Pena... google) is an Irish pub/happy hour until midnight bar. It doesn't open until ~5pm. Usually the bartenders are native English speakers, but the crowd is mostly Argentine.
Warsaw (in Monserrat/San Telmo) is a bar/pub/place I watched a Champions League game. The crowd there was mainly expat, but that's the only time I've ever been. It was mid-afternoon and showing a European football (soccer) match, so the fact that it was filled with expats then isn't that surprising. I don't know if it usually is.
our site is www.clubeuropeo.com and we meet in Corrientes 327 Piso 21, downtown. On Mondays we talk English and German, on Tuesdays also English and best day is Wednesday, where all the young commissions meet to talk, have coffee and beer. As from 7-7.30. Last Thursday we had a terrific After Office Party. Come and join us !