When I speak Japanese, I can speak twice as much compared to English.
When I speak English, I speak only half as much as I would if I were to speak Japanese.
1) When I speak Japanese, I can speak twice as much compared to English.
「日本語を話す時は、英語と比べて２倍は話すことができる。」
speak twice as much で「２倍（の量）話す」
2) When I speak English, I speak only half as much as I would if I were to speak Japanese.
「英語を話す時は、日本語を話す時の半分しか話さない。」
speak only half as much as ... で「…の半分しか話さない」
ご参考になれば幸いです！
おっしゃられている内容は、以下のようにも表現できると思いました(*^_^*)
I speak twice as much in Japanese as in English.
「私は英語よりも日本語の方が2倍の量を話します」
順に文の成り立ちを解説します。
I speak much in Japanese.
「私は日本語でたくさん話します」
これにas asを付けて、
I speak as much in Japanese as in English.
「私は英語でも日本語でも同じくらい話します」
2つ目のasの前にtwiceを付けると、2倍の意味になります。
I speak twice as much in Japanese as in English.
以上です。
少しでも参考になれば幸いです(*^_^*)
★★大阪カフェレッスン英会話講師 KOGACHI
日本 
