It's similar, but generally an employment certificate is shorter and confirms ongoing employment. Can be 1-2 pages; whereas a contract can be rather lengthy.
Documents need to be translated into Thai or English at MOST consulates around the world, whether an applicant likes it or not. The exception being a few European consulates, which take certain documents issued in the local language.
Pretty unlikely anyone would come to Thailand without knowledge of at least English and preferably Thai as well. They'd be completely unable to communicate with anyone and Thais would be scratching their heads at how can a foreigner not speak English?
In fact, it's probably better to apply in countries outside of where your employment is based in case they impose strange requirements like extra documentation related to your work, for instance in Myanmar, they're asking for proof of registration with government authorities or something to that effect, which is not something many people can prove. An applicant working for a Burmese entity but applying in Laos or Vietnam won't be asked for such documentation and thus has a higher chance of success.
Forget agencies. They charge an arm and a leg and are why consulates are making it more difficult to acquire a DTV. As long as you have 500K to your name and can make it look like you're getting paid regularly then you're halfway there.