A prayer is an action involving words, involving the mouth
if you "just think it" you don't necessarily actualy use the words
and you don't elevate the "work of your lips" so to speak
You must "say"
However, the main three-times-per day prayer in Judaism (Shemoneh Esreh) is done on the model of Hannah, the mother of Samuel --
"out loud" means just loudly enough for yourself to hear.
"...Now Hannah, she spoke in her heart; only her lips moved, but her voice could not be heard; therefore, Eli thought she had been drunken." (1 Samuel 1:13)
The Shemah people usualy say in a full voice though
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English vs Hebrew - it is preferable to learn ahead of time so that you say the prayers in Hebrew AND know what you are saying.
Otherwise - yes it is okay to pray in English.
In a reform synagogue in an English speaking country, even many of the public group prayers are in English
But there is a definite power in the Hebrew language itself.
Mystics say that every word of the prayers were worked out in detail because of the particular word's power, evocativeness, connection to Godliness...
You can certainly study enough to learn the one phrase of the Shemah and say it in Hebrew.
"Shemah Israel, Ad-nai Elo-einu, Ad-nai Ehod"
"Hear oh Israel, the Lord is your God, the Lord is the one God"
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NOTE: The actual Halakha (law) does not differ between Orthodox and Reform -- the difference is that in the Reform movement the Halakhot are just not viewed as "obligatory"