I'm assuming that the first ん, after て, stands for る. However, I simply cannot parse the ねん that comes afterward. How exactly does "what the hell are you saying" become 何を言うてんねん, or if my first assumption is correct, 何を言うてるねん?
This is a sentence in Osaka dialect and means "What are you saying?" The "un-contracted" form would be 何を言っているねん (I know this sounds weird; see below).
- 言うて is an equivalent of 言って, the te-form of 言う. It's widely used in western part of Japan, including Osaka. It's pronounced as ゆーて.
- ん is a sound change of る, which is a contracted form of いる. See this chart. This る can be turned into ん in very casual Japanese. In particular, this る almost always turns into ん before ねん.
- ねん is a contraction of nothing. ねん is ねん, a sentence-end particle which is almost specific to Osaka dialect. It's like よ, のだ, のか, のです or のですか depending on the sentence. See: ねん in the Kansai dialect and Usage of ~まんねん (関西弁)