I have an English site with a subset of the pages translated into
Spanish. However when I query google with site:[sitename], by far the
majority of the pages returned are the Spanish translations, with very
few of the English pages returned (which are the majority).

I would like to know if the way I have implemented the translations is
adversely affecting the ranking of the site and whether the results of
the above search are an indication of this.

The way it is implemented is as follows:

All English pages are in the root directory, with all the Spanish
pages in a subdirectory off of the root. However, although the Spanish
pages all contain translated content, I kept the page names in English
and the same as the names of the English equivalents. This had a
number of benefits in terms of implementing the site, but now I wonder
if Googlebot is assuming that because the page names are the same,
that somehow it is duplicated content.

Is it worth me changing the page names of the Spanish pages into
Spanish?

Thanks in advance to anyone who can help me with this.