There is a lot of odd stuff going on here.
The translator himself comments: “I think it’s more important that the English and Greek express the same idea, even where the English idiom doesn’t work the same way as the German.”
Here’s the source for the translations and the comment - http://www.letsreadgreek.com/phrasebook/greekenglishphrasebook.pdf. I presume the OP is simply making a Memrise course of this, so let’s not abjure his idiom.
But setting aside the language used, many of the translations seem not to quite sit well - and it goes beyond how natural or otherwise they sound in English. Just one example - “ἴθι χαίρων” (I am looking now at just one page of the Memrise - Level 7) does not mean “Be happy!” when it is used in Attic Greek. Nor is that the usual sense of “χαῖρε πολλά” (again, same page of Memrise).
Regarding “well, hello there!”: “ὦ χαῖρε” in Attic doesn’t simply mean “hello” (to which “χαῖρε” alone would be pretty close) but rather something a bit different. It is often (to try and find something closer in English) “hail” (cf https://www.dailywritingtips.com/the-multiple-meanings-of-hail/ “hail, fellow well met!”, which is pretty much it). It could be that the translator is trying to express that, or at least convey that the phrase means something more than “hello”. I’m not sure how exactly you read the tone of the “Well, hello there!” exclamation - if it’s the same as me then yes, obvs this has implications in English that the Greek does not have. 
I agree that “very nice” is a horrible translation of “οὐκ ἄχαρις”, and as discussed above, I think the problem goes beyond the English idiom. It doesn’t capture the sense correctly. Maybe you said all that needs to be said on that one.
Quite apart from all that, the Greek itself seems dubious to me sometimes. Just one example is “εὐψύχει!” for “farewell!” (and I am choosing that from the one page I am looking at, the same one as above). No one writing a book called “Sprechen sie Attisch” should be using “εὐψύχει” rather than the obvious and famous “ἔρρωσο”. (Nor does the writer offer “ἔρρωσο” anywhere else in the book, according to a search of the PDF.) I was doubtful of “ὑγίαινε!” too but at least it’s used before the 1st century AD (once; by Herodian in a pretty corrupt passage). It is possible I am missing something here regarding the author’s intent but I have found a few more such problems glancing through just a small selection of the Greek.
Besides all this, even if the book had perfect Greek and (somehow) perfect English translations, I’m not sure what the point is. Learning to say “hello” and “goodbye” is facile and is almost certainly not the key to your understanding Greek better. You (OP) talk about using Memrise to memorize useful things like principal parts - why spend your time on something like this?
For the record, I am not opposed to work like this book or to their aim; but in my view the matter of rounding off someone’s efforts at spoken Greek by supplying them with otherwise hard-to-reach colloquial phrases is something that should follow a very strong command of the rest of the language, not precede it. Ask yourself, simply, how often a lack of knowledge on any given point would be a barrier to understand the Attic you read.