Full Idea
I don't think it is true that all thought is in Mentalese. It is quite likely (e.g. in arithmetic algorithms) that Mentalese co-opts bits of natural language.
Gist of Idea
Mentalese may also incorporate some natural language
Source
Jerry A. Fodor (In a Critical Condition [2000], Ch. 6)
Book Reference
Fodor,Jerry A.: 'In Critical Condition' [MIT 2000], p.72
A Reaction
Presumably language itself would have to be coded in mentalese. If there is some other way for thought to work, the whole mind could use it, and skip mentalese.