grammar - Explaining Tense in Japanese

I find that Japanese is a weird language for its learners, especially for those who've already learned English. In English one usually uses two tenses: present and past, however in Japanese there are accomplished and unaccomplished tenses, and these look strange if literally translated to English. For instance:

わたしは日本に行くとき、ラジオを買いました。
Literally "When I go to Japan, I bought a radio."

Here, the English sentence has two different parts which use two unrelated different tenses, so it's obviously logically wrong.

Another example:

私は日本に行ったとき日本製のテレビを買う予定です。
Literally "When I went to Japan, I decide to buy a TV made in Japan."

And this sentence also sounds strange.

Can anyone explain how to think about Japanese tenses for sentences like these? I'm really puzzled by it.