The valid words will depend on the selected language when starting the game, as each language has its own dictionary.
Proper names, acronyms, prefixes and suffixes will not be taken as valid words. Some words may be accepted if they have another definition, like a proper name and adjective, for example.
Please keep the following in mind:
You can place letters to continue an existing word on the grid.
The words can be formed from left to right and from top to bottom.
You can form a word next to other as long as the words can be read both ways, from left to right and from top to bottom.