Resolved questions
How would you translate this sentence?

Well... those slang Japanese expressions are pretty difficult to translate into other language precisely.

The context basically shows the meaning like...
普段強気なクセに恋愛に関しては超ヘタレだからな。
You usually have a strong attitude, but in terms of dating (games,) you are really a wimp.

And vocabulary words,
- 普段 usually
- 強気 (having) a strong attitude
- クセに　but / even though
クセに is used for a strong contrast sentence. Often used to express a person/thing which has attributes that are strongly unbalanced. For example, このレストランの料理は高いクセにマズイなぁ (The food of this restaurant is expensive but tastes bad!) In this sentence, the attributes (expensive price and bad food) of the restaurant are not balanced. In addition, クセに is sort of a derogatory expression. So be aware that sentences using クセに sound very condescending.
- 恋愛　love/dating/dating game
- ~に関しては　in terms of ~
- 超 (ultra) super/ very emphasized
- ヘタレ wimpy (sort of a derogatory expression)