The town I grew up in had some German speakers in it, so I picked up some of that along the way. (For instance, I know how to say "turn off the light" and "very hot.") I studied French in college and lived in Paris for a year. So I am relatively comfortable in basic French - but my comprehension is better in Paris than Montreal.
In most other languages, I simply speak food. I can read menus in Italian and Spanish. I can watch a telenovela and guess at the plot - but I can't pretend to know what the actors are saying.
What does all of this have to do with editing? It points out that knowledge of a language isn't the same as full comprehension of the intricacies of that language.
I ran into this recently when I was in South America. We found very quickly that - for the most part - you don't throw any paper into the toilets. There are signs posted almost everywhere (at least everywhere that tourists might go) that depict through graphics what you are and aren't supposed to do. Often, these are accompanied by a message in English:
Please do not throw paper into the toilet.
Pretty self explanatory, right? But... well... not everyone understands the use of proper prepositions - and "into" can be kind of confusing - even for native speakers.
We often saw the also-correct:
Do not throw paper in the toilet.
We saw the not-quite-correct:
Do not throw paper in to the toilet.
And, once, we saw my favorite warning:
Do not throw paper to the toilet.
Of course, being an editor who was frequently running on little sleep, I really assumed that there should have been an explanation for that last one, such as "Do not throw paper to the toilet, because it has no hands and cannot catch."
Did it make it impossible for us to understand? No. And, honestly, we could figure out all of those signs. Nothing was so far gone that it left us not knowing what to do or not do. Why? Because each of the translators knew the mechanics of the English language.
But, as an editor, my job is to make sure what I'm working on is not just mechanically correct - it also needs to be grammatically and understandably correct to a native speaker. It needs to feel right to its readers, and not be filled with rough edges that readers have to try to sand off while reading.
(And, yes, the same can be said for American English and British English - or any different dialects of the same language.)
As with most other processes involved in writing a manuscript, there aren't shortcuts in this. If you want your book to reach its full potential, you need to go through each of the hoops - one round for the translation and (at least) one more to make sure that the translation is not only mechanically correct. (And each of those rounds should be done by a person specializing in that process. A translator for the translation. An editor for the edit(s).)
If you're ever not sure it's worth it, just remember all of those people who may be trying to play catch with a toilet.