When someone can speak a language very well, they are "fluent". What if you can only understand it?

Solution 1:

You could consider using passive speaker as defined in Wikipedia:

A passive speaker (also referred to as a receptive bilingual or passive bilingual) is someone who has had enough exposure to a language in childhood to have a native-like comprehension of it, but has little or no active command of it.

As it is not a broadly used term, you might have to add something after saying it for example:

He is a passive speaker in French. He does understand it very well, but can't speak it fluently.

Solution 2:

I'm not sure that there is a generally accepted word for this, though in the book 500 Years of New Words: the fascinating story of how, when, and why these words first entered the English language, author Bill Sherk wrote:

The author of this dictionary coined the term sesquilingual in 1975 to describe people who know one language and part of another, a term that probably applies to the majority of Canadians, who know English and a smattering of French, or vice versa. The Latin prefix sesqui- is sufficiently unfamiliar to most people that if you describe yourself as sesquilingual, they'll often think that you speak 6 languages instead of one and a half.

Quite so. The OED definition of the prefix sesqui- is:

Denoting one and a half

Accordingly the word sesquilingual is not a bad description of this situation (the possessed "half" in this case being the ability to comprehend and the missing half the ability to speak), but it has the disadvantage of being effectively a neologism and therefore it's not at all certain that your audience will understand what you mean if you use it. (Not without doing a web search, anyway; such a search will generate a decent number of hits pointing to this definition.)

Solution 3:

As mentioned in a comment, "most anything you might use will require explanation".

To describe what you mean in a succint way, the best I can come up with is "good listening comprehension but poor oral production"