みんなの日本語 中級I - Lesson 3 says that V (dictionary form) ことにする is used to express the decision to do something. If I say
私は日本へ行くことにしました。
this can be translated as “I decided to go to Japan”. This is perfectly clear to me. However, what is the correct translation for
私は日本へ行くことにします。
? Is it “I will decide to go to Japan”, “I decide to go to Japan”, or “I decided to go to Japan”? Can you explain to me when to use する in non-past tense?
We don't have a perfect parallel structure in English. The deciding is not in the future tense even though the action that follows is.
I would go with:
I will go to Japan.
At least for me, the most common type of the present tense にする that I hear is for ordering food.
らーめんにします。
=
I will have the ramen.
In both cases, I take will not to be the future tense but the volitional will. You could replace with choose
「する」is a generic verb. It's like "do" or "execute". Your second sentence is like saying "I do go to Japan". As you probably already know, if the speaker just want to tell the plan, the speaker says 「私は日本へ行きます。」. If I hear someone says 「私は日本へ行くことにします。」, I feel that the speaker want to tell listener(s) that he/she gave thoughts on something before saying it. So, for example, it's weird if someone says it to a travel agent.
Conversation example:
A (to B): 「それで、どうするの。」
B (to A): 「会社を辞めることにします。」
next scene
A (to C): 「Bは会社を辞めます。」 or 「Bは会社を辞めるそうです。」
(A unlikely to say「Bは会社を辞めることにするそうです。」)
C (to A): 「分かりました。」
