Nowadays all mobile phones allow you to select Japanese from settings/languages. You should be able to do it. I've done it with mine that is an android. So on my keyboard now I've got English, Spanish, Japanese and Italian. I switch from one language to another just pressing the language key.

there's is an application, go to Google play store (don't know about the iPhone but you just have to look)called "Google Japanese input"

https://www.youtube.com/watch?v=u3ZuELh7rhI There's a useful video in the YouTube.

You just have to write the Japanese word in Romaji, then select the kanji or katakana spelling from the drop-down menu.

Nice too meet you