Learning stuff from phrasebooks or apps is useless. It is hard to remember it when you don't understand the basics of the language.
It's also hard to learn katakana and hiragana if you are not actively using it. I am not learning kanji formally but have picked up several hundred just by observing.
One thing you can do is to use Japanese word order when you speak English or try to speak a sort of pidgin Japanese. Subject object verb. And if you don't know the verb then create one by XXXX-shimasu. This is done with borrowed verbs (a lot of "Chinese") verbs and examples like Kyanselu (cancel)-shimasu even there is a Japanese verb for cancel. One thing that you have to remember is that everyone in Japan supposedly studied English for 6 years in school. And the language has a lot of English loaner words.
But then the question is what use is it when they start rattling off in Japanese. They typically do not speak to you in simple terms or adjust their language.
Also if you deal with service industry people then they won't speak with words that you learn in a intro textbook because they use humble and honorific forms.
p.s. Imiwa is indeed a great ap. It's Iphone only though. For tourists a nice thing is that it does not need a data connection. It's a large file like around 500 MB. But it is more useful for people who have some proficiency.