First, please look at this sentence:
私は日本へ行く前に、中国のお土産を買った。
Its English translation is:
Before I will go to Japan, I bought some Chinese local goods.
which is very weird.
It is said that Japanese tense has relative tense which does not exist in English.
Can you explain that for me?
私は日本へ行く前に、中国のお土産を買った。
Before I will go to Japan, I bought some Chinese local goods.
The example sentence might be better translated this way:
OP - "It is said that Japanese tense has relative tense which does not exist in English."
Perhaps what is meant by "relative tense" is that the verbs in the two clauses do often need to agree in tense in English, but they don't need to be in the same form for Japanese, in fact that would be incorrect for this sentence.
However, the primary difference between 行く and 行った in Japanese is not a difference of "tense"（時制）, it is a difference of "aspect"（相）. Aspect is affected by tense, but they are not always equivalent.
In other words,
If you look at the Japanese example sentence again with this viewpoint it should make more sense.
私は日本へ行く前に、中国のお土産を買った。
These articles may be a good reference:
As mentioned by @YOU, there is nothing wrong with the translation
Before going to Japan, I bought some Chinese souvenirs.
Tenses in Japanese are relative to the tense of the verb of the main clause. If the main action is in the past, then past tenses in the relative clauses imply that those actions took place at a time earlier than that of the main clause.
In this case, the main tense is a past tense and the tense of the relative clause is non-past, which implies that this action has taken/will take place at a time later than that of the main clause.
