speaking (in) Spanish

[nyota]

1. Neither of my parents speaks English. = knows and uses it habitually
2. The guide speaks English, German, and Spanish but he'll be speaking in Spanish this afternoon because most of the visitors are from Spain. = use the language to communicate on a particular occasion

I've taken it from Longman's Dictionary of Common Errors. Is it a strict rule in 2 or could I omit the preposition and still sound right?

[5jj]

It's fine without the preposition, in my opinion.