[Nyororin]

I say not to do it in Japanese.
If you`ve been communicating in another language all along, using Japanese for just this one event may actually have the opposite effect and not feel real at all. I know that if my husband were to say words of love in English (which is probably at the same level as your Japanese) it would be the strangest thing. Definitely not the feeling you want when you propose. If he`d proposed to me in a language he wasn`t that good in - I doubt I`d have believed it and would have required a confirmation in Japanese.

Tell her you love her and want to marry her in the language that carries meaning for you and for the both of you. That is what matters.