Inspired by this. My understanding had been that only the Revised edition rules were available in English, and that a translation of the Core Exxet (2nd ed. core rules) had never made it into English. Other than Time Travel, has anything else changed?
The "English second edition" is more of a revised printing that includes the errata, but gets mistaken for a second edition anyway. There is an even newer Spanish edition/revision, but it does not appear to have been translated yet. It's worth noting that the publisher does not consider it a new edition, rather a revised edition, because it will be compatible with existing material. I'll call this the "2nd revision", to distinguish it from the first errata-only revision.
So for the two English releases, the only difference is that the second includes the errata.
As for the latest Spanish-only revision, according to the publisher's statement at the time it was announced, it doesn't change the game substantially, but tweaks some rules. An English summary of the relevant parts (the announcement was in Spanish) was made in an RPG.net forum thread:
The base isn't altered, but there are several modifications to the rules.
Abilities: several reworked, more abilities (or more usefulness) for less development points at creation, better explanations with difficulty tables and examples
Attribute controls: completely revised
Magic system: probably the most radical change, 4 levels of power for every spell
Combat table is out, but the same options in combat remains (more dynamic and quick combats, they say)
Weapons and attacks, damage system remodeled
So, meaningful but small-scale changes.
Notably, the perfectionist cataloguers at RPGGeek agree that the game has only one edition so far, as they don't list the English "2nd edition" as a new edition, just a revised version of the same item. There is no database entry yet for the Spanish 2nd revision. The release dates also tell the tale: the Spanish announcement of the second revision incorporating new mechanics was in 2010, and there has been no new English version of the core rules since then.