*Would France ever stop seeing Germany as a threat? If so, when?
When something else becomes a bigger threat.
*Would Germany's reparations ever be cancelled outright?
They would likely have been put on hold as per OTL and assuming the planned 1932 rearmament plans were put in place its likely they wouldn't have come back and the Germans default on the US loans until they could stabilize their economy.
*Would unification with Austria ever happen without the Nazis in power?
No; they tried that with the Austro-German customs union, but the French pulled their money out of the Austrian banking system in retaliation and collapsed both the Austrian and Germans banks, leading to the rise of Austro-Fascism and ties with Mussolini. Absent Austrian Hitler the Prussians and 'regular' Germans in charge in Berlin would focus only on Poland and not really care about Austria. Austria in the 1930s was more of a burden and underdeveloped compared to Germany, so it wasn't a desireable option unless you were a pan-Germanist, which the Prussians were not.
*How long would it be before Germany fully accepts Polish independence and stops having designs anywhere east of the Polish border? How would German-Slavic relations be in this scenario?
Depends; if they cannot force some border renegotiations then probably 1950s-60s at the earliest IMHO. The old school Prussians need to be filtered out of power. Also the rise of the USSR as a threat might do it, as it would leave Poland as a critical buffer. Hitler was the guy that normalized relations with Poland, so likely frosty relations remain for a while.
*The Soviet Union in this scenario?
Probably a good trading partner with Germany, much like Russia is to Germany today; they trade raw materials for German industrial goods and both sides profit. The Soviets continue to rise, Stalin continues to purge, and the USSR suffers under Stalin even as they grow economically and militarily.
*What would relations be like between Germany and the UK/US?
Probably better between Germany and the UK especially as trade picks up and the Great Depression recedes. They probably divide up markets like IOTL, Germany rearms a bit to 1960s Bundeswehr levels, and it is Britain's anti-Soviet proxy on the continent as it starts to dominate markets and overtakes France decisively. The US-German relations would be frosty because Germany has no choice but to default on their US loans for a while until they stabilize their economy and balance of trade; the US probably after 1936 negotiates better terms and trade deals to help Germany get back into the financial system on good standing so that they pay what they owe. By the 1940s a non-extremist Germany is actually in pretty good graces in the US as they pay back their loans.
It would sort of be like German-US relations today without NATO or Germans being tainted by WW2 and the Holocaust. German-Americans wouldn't be ashamed of their heritage and the German language and culture would still the 2nd only to English in the US and globally. Especially as the German economy and political position recovers it will be much more favorably looked at by the world, especially within Europe as a counter to the USSR (not in France or Poland however).
*How important would German be as a second language on the international stage? Would it still become less important vis-a-vis English?
It would be the counter to English and anglo-phone domination of the globe. It would certainly be 2nd, but much closer than any other language you could imagine, as Germany would be much more powerful economically relative to the US and still be the language of science and social sciences. German cinema would still be the counter to Hollywood, but the US would still pull ahead, just not at fast or as far. Especially if German birthrates don't collapse as they did IOTL and without the huge death toll of OTL Germany would be quite a bit more populous, prosperous, and cultured (the Nazis slaughtered German high culture when they got into power in all manners, not just by exiling German Jews and Liberals).