Corrected capitalization, corrected to American spellings and typos (#30685)

* Translate challenge subtitles and example challenge text to Spanish

* Corrected errors in syntax and punctuation

* Multiple corrections of it/s to its plus other grammar corrections

* Correction and added paragraph to CSS Flex article

* Corrected my own typo

* Corrected capitalization, American spellings and typos

guide/english/javascript/loops/for-loop/index.md

@@ -13,7 +13,7 @@ The javascript `for` statement consists of three expressions and a statement:
 
 ## Description
 
-*   initialization - Run before the first execution on the loop. This expression is commonly used to create counters. Variables created here are scoped to the loop. Once the loop has finished it's execution they are destroyed.
+*   initialization - Run before the first execution on the loop. This expression is commonly used to create counters. Variables created here are scoped to the loop. Once the loop has finished its execution they are destroyed.
 *   condition - Expression that is checked prior to the execution of every iteration. If omitted, this expression evaluates to true. If it evaluates to true, the loop's statement is executed. If it evaluates to false, the loop stops. 
 *   final-expression - Expression that is run after every iteration. Usually used to increment a counter. But it can be used to decrement a counter too.
 *   statement - Code to be repeated in the loop