Before I will go to Japan, I bought some Chinese local goods.
The example sentence might be better translated this way:
- "Before going to Japan, I bought some Chinese souvenirs."
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,
- 行く describes an incomplete action (which may take place in the past, present, future or even habitually), but...
- 行った describes a completed action (which may take place in the past, present or future)
If you look at the Japanese example sentence again with this viewpoint it should make more sense.
- 行く前に - "before going" (not complete because 行く is describing a point "before" the action of "going" takes place)
- お土産を買った - "bought souvenirs" (complete because the speaker has already completed the action of buying)
These articles may be a good reference: