Wa Alaykum Assalam and thank you for your question.

Yes, you may make any dua in your own language; Allah understands all languages.

Many Muslims memorize prophetic duas in Arabic then learn its meaning in their own language to understand it and it also helps them memorize it better. It is also advisable to ask someone who knows Arabic to teach you some short Arabic duas.

However, it is not an Islamic requirement that you learn the duas in Arabic. It is only recommended because we will be saying exactly the same words that the Prophet (peace be upon him) had said to invoke Allah.

Obviously, understanding is better than memorizing without understanding because it increases our level of sincerity.

Therefore, you may read the istikhara or any other dua in English. It is your sincerity that will make the difference.

Try learning short prophetic duas in Arabic just to improve your language skills; I think it will benefit you in the long term.