These sentences are examples of hypothetical conditionals, also known as second conditionals.
Rearranging your first sentence to lead with the if clause, this is what it looks like first as a simple conditional and then as a hypothetical coditional.
If there is only one language, [it] will all be lost
If there were only one language, [it] would all be lost
A hpothetical conditional requires a subjunctive in both clauses. The only verb that has a distinct subjunctive is be where the subjunctive is were. For all other verbs, we simply backshift the tense, as we do for reported speech.
In informal spoken english, we can use a backshift instead of the subjunctive for be, so this sentence would be OK:
If there was only one language, [it] would all be lost
To answer your first question, then, you cannot use there is because it breaks the pattern for a hypothetical conditional, but you can use there was in informal spoken english.
Rewriting your second example in simple and hypothetical conditionals:
If more people strive in life toward true happiness, the world will be a better place.
If more people strived in life toward true happiness, the world would be a better place.
There is only one valid backshift for will, and it is would. The other options are not valid in this particular sentence.
The would have option would work if you were talking about a hypothetical past- what whould have happened if people had done different things in the past. Note that you have to change the verb in the first clause too.
If more people had strived in life toward true happiness, the world would have been a better place.
THere is no sentence in which would had been is valid.