While potentially a laudable initiative, it suffers from two major problems:

1. Suggesting a subreddit dedicated to bilateral relations between two closely intertwined communities while suggesting English, which has nothing to do with either culture, as a standard language of a cultural subreddit, raises issues with many folks here. In a way, it can be considered paradoxical as you speak about the suppression of Galician language and culture by an outside culture, and then suggest an alien language to both as subreddit language, which further contributes to the supplantation of both Portuguese and Galician as languages of communication over the internet, and even over communication between these two intelligible languages.

2. Adding the political motivation of unification of Galicia to Portugal; nowadays Galicia is extremely Castillianized, so what support might have historically existed, is no longer materialized. Further, while Galicia was historically known to be an extremely poor region of Spain, nowadays it is quite not so. In fact, in terms of hard economic data, it is richer than the overwhelming majority of the Portuguese regions. So any economic allure there might have existed historically, it likewise, is no longer in existence. Finally Portuguese in general don't particularly want for Galicia to join the country. We have done our path separately for close to a millennium now, and right now it makes little sense to change that. So you have a politically charged proposition that neither Galicians so nor Portuguese want.

Those are major reasons to why you're getting a lot of resistance to your subreddit idea.