Are there any specific differences in translating apps as opposed to other written documents - for example, does the use of idioms make things more difficult?
Of course there are differences. Translating apps is different from translation a plain text document. In both cases, there will be always the final users/readers, but it is different.
For example, if you translate a tech related article, you will have 500, 1000, 2000, etc... words for translation. Without interruption. Sentence followed by sentence. Translating an app, even if you have a total of 500+ words, will always be different, because usually those words are separated in smaller sentences and strings.
Perhaps you can say it's easier to translate an app than a plain text document. Yes, probably it is in some cases. But you can't compare them.
I would say that translating an app would require some technical knowledge, and perhaps some insights about the actual world of apps. While translating documents, you just need to know well the subject.
Hope this answer was useful.
Thank you! It certainly was!