time series plots: labels language & tickmarks

My native language is spanish and I would need to do two changes in the
default xlabels in timeseries plots:

What sort of plots are you talking about here?  (Not tsplot or plot.ts, 
for example.)  I think you are perhaps talking about plots of Dates.

a) For timeseries that span more than one year, there are just xlabs and
tickmarks for the beginning of each year. Could I add extra tick marks for
each month easily?

See ?axis.Date, if this is about Dates.

b) The xlabs appear in my language for months and days (spanish) but I need
them in English, how can I change this setting?

Use R in an English locale.  (If as the posting guide asked you have told 
us your platform we could have told you how.  If you had shown us some 
reproducible code we would not have had to guess as to what you want to 
do.)

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595