Is it secure to change these options on working production system?
(System Configuration)
Default Frontend Language - English
Use language character map - No
"Use language character map" will only affect new assets. It looks at the new web path , and do the character replacement.
So it should be safe to enable that.
"Default Frontend Language" is normally en, if you change that, it might affect the existing asset's frontend language setting.
e.g If you use keyword %asset_language% to print asset's default frontend language, and there is no override language specified for the asset, it will always use "Default Frontend Language".
So the change could affect that. But I'm not sure the exact effect of the change, it depends on implementation. It might not affect anything at allif you don't use that keyword.