@aral
Weblate or Transifex appear popular (although the latter is deputed in FOSS circles).
For implementation i18next is quasi standard. Native desktop apps might go with plain old gettext.
Watch out for handling adjustments to amounts (not as simple as in English) and the ability to pass comments (otherwise „share“ of content might get translated as „share“ in context of stock market ).
Mozilla had something with l20n but I haven't followed along.