It's wordy, but you see what I mean: I think "seu" here refers to either the llengües natives or the immigrants actuals. A basic interpretation:

"The situation of Catalan language immersion differs now from twenty or thirty years ago, since although Catalan is used in classroom materials, there is enormous language diversity in the current immigrant population. What's more, this diversity does not extend into society at large, but rather remains in the milieu of its sources."

or: the neighborhoods of its sources [here the speakers]

Everything changeable, of course. But I hope this helps!

"....without any social weight outside the home."

NOTE: I read this as meaning that the immigrant's language is likely to have little weight outside the home (few people speak Arabic, or Serbian in the streets of Manresa :-). However, in the subtle ways in which these things are often stated, I think the "echo" here is that the immigrant is far likelier to be exposed to Spanish in the street than Catalan.

If I am right in my reading between the lines, I can tell you that while this statement may be true in the larger urban areas of Catalunya, the situation is not so simple in the smaller towns and villages of the interior, which are now receiving significant numbers of immigrants and which are often largely populated by catalanoparlants.