In English I would write
Lukas’s table
and pronounce this as Lukases table, i.e. with an additional s at the end.
In German, I think it’s written like this:
Lukas’ Tisch
without the additional s after the apostrophe. How do I pronounce this? Shall I pronounce it without the additional s as Lukas Tisch or with the additional s as Lukases Tisch?
The answer is:
[ˈluːkasˈtɪʃ]
This means: You just pronounce it as if there was no apostrophe. (The apostrophe in the phonetics means that the vowel in the following syllable is stressed. It has nothing to do with the apostrophe in the normal written term.)
Hubert gave the correct answer for at least partially formal speech, where the fact that a genitive is expected is known and no confusion can occur.
However, you should always remember that genitive case is not the most common case in colloquial speech. For example, as I jokingly commented, Bavarians (including Austrians, and likely also Swabians/Alemannians including Swiss) would tend not to use any genitive at all. Instead constructions such as the following would be used:
Dem Lukas sein Tisch.
This is even more notable if we put this all into a conversation:
Wem gehört der Geldbeutel, der hier liegt?
Das ist dem Lukas seiner.
(or, for comparison, see below:) Das ist dem Max seiner.
In the North of Germany, the genitive case still exists somewhat even in colloquial language, to the point where constructions such as the one in the question (‘Das ist Lukas’ Tisch’) can be heard in everyday language. In that example sentence, it is clear from the context that we are expecting a genitive, so all is well, the pronunciation is as it is written and as Hubert noted. (At least assuming that both sides know they are talking about a table and that Tisch is not Lukas’ last name.)
In cases where it is not necessarily fully clear that a genitive is occuring, a number of different, non-standard and non-standardised clarification methods exist, similar to the English pronunciation of Lukas’ being essentially Lukases. For example:
Wem gehört das Portmonee, das hier liegt?
Das ist Lukasens.
(or:) Das ist Maxens.
In formal or written language it should still be Lukas’/Max’, but in spoken language it can get extended as shown for clarity.
