義理チョコをネイティブスピーカーにわかってもらうためには、どのように説明したらいいですか？

「義理チョコ」を一言で訳すと、”obligatory chocolate” あたりかと思いますが、これだけでは何のことやらわからないでしょうね。そもそも、バレンタインデーそのものは欧米からの輸入ですが、女性が男性にチョコレートを贈ると言うのは日本の特殊な風習と思います。

例文：

In Japan, it is customary for a woman to give chocolate to her lover on Valentine’s Day. Actually, this has led to another custom that women often give chocolates to their male coworkers or bosses on the same day, feeling somewhat obliged. That kind of gift is called ”Giri-Choko”, meaning ”obligatory chocolate”. You can find many kinds of ”Giri-Choko”s on display at shops in the season. Usually they are nicely packaged as gifts. But they are not too gorgeous, in order to avoid confusion with ”Honmei-Choko”, that is the ”real” chocolate showing love.


　

In Japan, girls give boys chocolate on Valentine’s day to show their love. The chololate often means declaration of love if he is not her boyfriend yet, and the cholocalate means confirmation of love if they are already a couple.
However recently girls/women start to give chocolates to their male co-workers/customers to show their daily appreciation. It’s nothing like the ones I describe above, but more like a greeting.
Now there are two kinds of cholocate, Honmei-choco (chocolate for someone special) and Giri-choco (cholocate for someone who you have to socialize with).
Usually Honmei choco is expensive and special, normally with a card. Giri-choco is cheap simply because girls have to buy many boxes of Giri-choco at workplace and they don’t want their co-workers misunderstand that the cholocate is Honmei.
Basically Japanese people love to exchange gifts for socialization in every occation and that is one of the reasons that the Valentine’s gift ”evolves” this way.