Missing is easy - just have enough developers working on the same thing, there's bound to be some differences in how you do things. Also, best practices and implementations change during development, so it's easy enough to understand how some strings were tagged for localisation and some not.
Testing is actually what brought the issue to HBS' attention, so I think it's rather unfair to hold that against them.
Finally, the "promise": Here's what the Kickstarter campaign stated (and still states):
"The game will also be made available in German, French, and Russian (with localized text, and audio subtitles) in a post-launch update."
- https://www.kickstarter.com/projects/webeharebrained/battletech
- https://www.kickstarter.com/projects/webeharebrained/battletech
Indeed, I've been involved with localizing games since 1995, including many sim-ship situations. One thing that I can tell you from experience is that no two game development productions are the same - they differ in complexity, people, tech, ambition, resources, budget, time, pressure, changing market landscape, management, first iterations vs expansions or sequels, and on and on...many factors can make things easier or harder. Taking on a new title as ambitious as BT, with as many constraints as we had ("punching above our weight" as we sometimes say), I would echo what 
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