This is both a question of Jewish law and of common sense. The sages have already made clear that one may pray in any language, as long as one understands that language. The exception to this rule is Hebrew, which has unique spiritual efficacy. One may pray in Hebrew even if they do not understand it. Hebrew is the language that the Torah is in, and which the sages composed the prayers. Everything else is a translation.
So Jewish law truly leaves it to the preference of the worshiper.
Enter common sense. A four hour prayer service spent being mystified by language that one does not understand can be a painful experience. Certainly one should preferably utter prayers in English that can be said with sincerity and clarity. On the other hand, to completely disconnect from the Hebrew prayers in progress is to be somewhat distanced from the community. There must be an appropriate formula for compromise.
I suggest the following: the recitation of the Shema, the Jewish proclamation of faith, should be read in Hebrew. All of the Congregational singing and responses should be done in Hebrew. One should join together with the congregation as much as possible.
The silent devotion can be done in English. It is, after all, the centerpiece of all prayer. I feel strongly that knowing what one is saying is the only way for a true religious experience. Similarly, some of the liturgical poems that the congregation recites silently should be said in English if one does not understand the Hebrew.
We must always keep the main goal in front of our eyes. That goal is to have an uplifting holiday, where the synagogue service is filled with beauty and meaning. Most synagogues will have a Cantor with a beautiful voice and a fine selection of melodies. People will be dressed well, the sanctuary will be decorated for the holiday. The congregation will be friendly and welcoming. All of the elements will be in place for a fantastic religious experience. Our goal must be to use that to transform ourselves for the better.
The best thing I can suggest beyond all this is preparation. Take some time to familiarize yourself with the holiday prayer book. Read through some of the prayers in advance. Learn their history, understand the structure of the synagogue service. Know what is happening in the silent devotion, in the Cantor's repetition, at the Torah reading, at the shofar service and in Musaf.
Saturday night we begin the high holiday season with the Selichot midnight service. It's a great opportunity to become accustomed to making prayer meaningful. I wish you all much success and happiness in the coming year.
