When singing opera in a foreign language, from my research, it is important that the audience can either see surtitles or translation a in the programme. For a concert, it is particularly important to give the audience information about each opera, as they will not be taken through the whole plot.
However, trying to explain all the twists and turns of an opera such as The Marriage of Figaro in a paragraph may be impossible. Therefore, the circumstances surrounding either the particular character through the opera, or the circumstances around the aria would equally be useful.
For example, for Musetta’s aria in La Bohème, it is not necessary to give the plot involving the characters that the audience will not meet in the concert, so it would be appropriate to simply mention the setting and what she is doing:
In 19th century Paris, Musetta is a coquette, and in this aria she showing off, trying to attract the attention of her former lover.
Each singer needs to assess their aptitude for verbal introductions. The research indicates that audience members like the introductions to be executed well, or not at all. There will be some audiences who are more knowledgeable than others on opera plots, but it is usually best to make no assumptions about audience knowledge so no-one feels excluded from the outset.
It may not be necessary to introduce every aria in turn. It can be effective to open the concert with an aria, and then to explain what it was and what the next aria will be. Arias can also be introduced as a group, which also helps the audience to follow the narrative of the programme.