昔はよくプロ野球の試合を観に行きましたって英語でなんて言うの？

*I often watched the baseball league.- Often means frequently.

*I'm a fan of professional baseball. -A fan is a person who supports sports, person.
A: Jack you like baseball so much.
B: Oh yes I breathe, eat and sleep baseball.

「発生率」か「度数」に関する話では、英語のネイティブ・スピーカーは誇張されている表現や単語をよく使います。
例えば、「よく（often)」の代わりに「いつも（always か all the time）を使い、
そして「あまり〜しない（rarely, hardly ever)」という意味を伝いたい時は「全然（never)」と言います。
例）I never go shopping
は
「あまりショッピングに行かない」
という文です。
"I used to watch professional baseball games all the time."
"I always used to watch professional baseball games"
はただ、「昔はよくプロ野球の試合を観に行きました」ということです。

These example sentences above make clear that you are attending baseball games in person -- not just watching them on TV. If you were always at home on the couch, you might say

-- I used to watch the game on television every night
-- I loved to sit on the couch at home and watch the game.

The 'ballpark' is the official name given to the stadium ground baseball is played on. As the majority of stadiums are for professional use only, its obvious that you are talking about watching professional games.

'Pro' is the abbreviated way to say 'professional'. You could say that a professional baseball player was a 'pro baller' which is slang for 'really good at something', and it can be used outside of a baseball context too.

To say 'all the time' does not literally mean you spent all of your time watching the games, but it suggests you went frequently.

Explanation: You may use the phrase, 'in the past...' when talking about an earlier occurrence. 'Frequently' is an adverb with similar meaning to 'regularly.'
Example sentence:
"In the past I frequently went to church, but not these days."

If you use to go to watch professional baseball games often and you wanted to explain this to whomever you are speaking to you could state one of the following: "I used to watch professional baseball games often", "I use to often watched the baseball league as I enjoy them" or "I use to love to watch professional baseball games".