I'm still looking for a solution but I can't find it.
I think the condition logic is always "or" rather than "and" so what you are asking is probably not possible without a separate set of conditions for the manuals text, i.e. English Installation, English User, Spanish Installation, Spanish user, etc. that you apply only to those strings.
In many cases OR condition is fine but in cases like this it doesn't work, and AND condition would do it. The main problem of your advice is that I would have to repeat a lot of work. For example, I would have to assign 'pounds' to 'USA Installation' and 'USA user' and 'kg' to 'Spain installation', 'Spain user', 'Italian installation', 'Italian user', 'French installation', etc. And that for every conditional text of the document. So tiresome.
I don't see why you'd need to change anything but the conditions on user manual/installation manual, unless those other terms only appear in conjunction with one or the other of those two strings. Leaving everything else as simply assigned a language condition ought to work. You would choose the language condition and the manual condition. Text assigned the language would appear, and text assigned the manual would appear.
Yes but I should have 18 labels:
English (for weights, references, and other stuff)
English Installation manual (just for text 'Installation manual')
English User manual (just for text 'User manual')
Spanish (for weights, references, and other stuff)
Spanish Installation manual (just for text 'Manual de instalación')
Spanish user manual (just for text 'Manual de usuario')
And I don't want 18 text conditions labels.
You could ask over in scripting, but I don't think what you want is possible. Perhaps someone knows about a third-party plugin that can handle "and" logic for conditions.