Or at least it wasn't originally. When I was working on Combat Flight Sim 2 we released 5 fully localized versions. Not just English, but FIGS (French, Italian, German, and Spanish). Since it was Microsoft, the process of localization was well defined. We did the easy things like using the correct locale for dates and numbers, and all of the UI and in-game text were resource based. We had 3rd party vendors to provide the resources and people trained in testing such things.
Of course, since word order can be different, not just that, but our strings were tokenized as well. We didn't just localize the words, we localized the format strings so we could but do word order and gender matching as needed. We'd just pass in enough strings (which were localized resources themselves) to the formatter and it would pick and choose which ones to use. And we did the same thing for our sound UI. Radio communications were done as voice instead of text, so we had to do the same kind of audio buildup. From a programming standpoint that wasn't much different from the text, but we did put a big load on our sound team. They had to make sure all of the snippets were done in a way that they could be strung together in some random order and sound right. That was a huge combination of strings, and my hat's off to them. Somehow they were able to do it.
All of that was relatively straightforward. The place we got bit was more cultural. As is typical, we did our UI wireframes in English. And there were cases where our UI got dense. Orders of battle, post action reports, pilot statistics. And our designers made it look good. There were several places where we had "Tank(s)" in the UI. And knowing that some languages use longer words than English, there was room for some extra characters. Then the localizers got involved. We got back the new resources, plugged them in, and tried it out. It worked, but the visuals, not so much. Turns out that instead of "Panzer", which we expected, we got back "Panzerkampfwagen" (armored combat vehicle). That's correct, but it didn't fit. Germans also don't like acronyms. They tend to make up new words out of groups of words, so we had to deal with that as well. In some cases we had to re-work the UI. In others we used modern words (like Panzer) instead of the more temporally appropriate terms.
So the moral of the story? Be flexible, Be adaptable. And remember, not everyone does things the way you do.