I just wonder if it is legal (according to the OGL) to summarize/translate the key rules of the 5e SRD in another language (say German or French) using the OGL?
As long as you satisfy the requirements of the license, the OGL specifically grants permission to translate open game content.
From section 1, definitions part "g":
"Use", "Used" or "Using" means to use, Distribute, copy, edit, format, modify, translate and otherwise create Derivative Material of Open Game Content.
And section 4:
Grant and Consideration: In consideration for agreeing to use this License, the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to Use, the Open Game Content.