URL structure for multi-language site

I want to make a multi-language site, but I'm not sure what the best way is to deal with the URL structure from a SEO point of view. Some examples:

Product

example.com/product/123/en/product-name

or

example.com/product/123/product-name/en

Or other?

Category

example.com/Category/en/furniture/chair/

or

example.com/Category/en/120/chair/

or

example.com/Category/en/chair/

Or other?

(Ignoring the SEO aspect, as it’s off-topic on Stack Overflow.)

Having the language tag as the first path segment is the most common convention. And it makes sense, especially if you translate the path (which you should!), because the path represents a hierarchy:

The top entry is the homepage:

• example.org/en
• example.org/de

From there, you select a product:

• example.org/en/product/123/chair
• example.org/de/produkt/123/stuhl

Or a category:

• example.org/en/category/furniture
• example.org/de/kategorie/möbel

If having the language tag somewhere in the middle or at the end of the path, it wouldn’t allow for browsable URLs (at least not in a sensible way, with translated paths).