Just so you know, you don't really have to bump around here... we're pretty good at getting to posts even if they sink down a bit.
For simplicity sake, this example changes your get to $manufacturer (be sure to clean this input before you run the query or there could be trouble).
To join them with a LEFT JOIN (which is probably what you want), you'd do something like this:
SELECT p.id_product, l.name
FROM pl_product p
LEFT JOIN pl_product_lang l
ON p.id_product = l.id_product
WHERE id_manufacturer = '$manufacturer' AND id_product = '$prod_id'
Wrap that in "" and you won't need to escape them and use string concatenator (.) and all that ("" process simple variable names in them automatically, very handy).
So, basically to do a join you start with a normal query. Then you figure out what table(s) you want to join. You then specify them in the join (LEFT JOIN is the most common, though there are about a dozen different types which you can check out in MySQL documentation). After that, you then specify an ON condition, which specifies which value in table A matches a value in table B.
After that everything else is pretty much the same.