Rather than subtitles, after establishing there was a common language, they could just have shifted to English, like they did. They could then lampshade it later. Have someone new walk in, only to find them all talking in galactic. O’Neil apologising, then shifting back to English, when talking to a new politician or diplomat, could have been some amusing character building.

The plot hole is filled (they are all talking galactic standard) while the viewer are happy (since it actually in English).