"Ble mae Siân? Yn y gawod eto mae'n siŵr."

Translation: Where is Siân? In the shower again probably.

[y_ddraig_las]

siwr = sure, doesn't it? not 'probably'

[EllisVaughan]

In this sentence it is best translated as "probably" or "most likely".

[lsc3994]

How about "surely"?

[EllisVaughan]

I would say yes normally, but it feels a bit awkward in the English sentence. Either way it's worth reporting it as a suggestion.