If you have some commonly used words that aren’t recognized by the standard iPhone dictionary, you can add your own words to the dictionary, it just requires a little “trickery” to get there. For some reason, Apple didn’t directly build in the ability to edit the iPhone dictionary, but it’s easy enough to add without having to hack or jailbreak your phone.
First, navigate to Settings > General > Keyboard. Once here, you should see that you have 1 (maybe 2 depending on if you have emoji enabled) International Keyboards.
Once here, click International Keyboards > Add New Keyboard. Then find Japanese Ten Key.
Now click the home button, then go back to Settings > General > Keyboard. You should now see the Edit User Dictionary option as specified below. You can click into there and add all your own words, terms, abbreviations, etc.