Hi, Edmond,
Is it just the blog content you’re looking to translate or the entire site?
If it’s the entire site, you may consider putting all of your English content under a /en/ subdirectory. For example: http://yoursite.com/en/englishcontentgoeshere.
As far as the blog by itself goes, I think you would be able to employ the same structure.
Alternatively, you may consider putting all of the English content under a Category called “Content in English” or something similar. This is probably the simplest approach.
One important thing to consider is your target market. Are you targeting English-speaking audience in U.S. or in other countries? Where is your Spanish-speaking site based at and who is your target audience? You will need to plan for that and localize accordingly.
Vic